[style](trx-ftx): fix clippy question_mark and collapsible_match

CI runs a newer clippy (1.97) than was available locally, which flagged
three lints in trx-ftx not caught earlier:

- question_mark: replace the Some/None match in CallsignHashTable::lookup
  with `self.entries[idx].as_ref()?`
- collapsible_match: fold the nested `if` in text.rs char/nchar into match
  guards on the AlphanumSpaceSlash arm

Behaviour is unchanged; verified clean with nightly clippy (1.93).

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

src/decoders/trx-ftx/src/common/callsign_hash.rs

@@ -160,18 +160,14 @@ impl CallsignHashTable {
         let mut idx = start_idx;
 
         loop {
-            match &self.entries[idx] {
-                Some(entry) => {
-                    let stored = (entry.hash & HASH22_MASK) >> shift;
-                    if stored == target {
-                        return Some(entry.callsign.clone());
-                    }
-                    idx = (idx + 1) % CALLSIGN_HASHTABLE_SIZE;
-                    if idx == start_idx {
-                        return None;
-                    }
-                }
-                None => return None,
+            let entry = self.entries[idx].as_ref()?;
+            let stored = (entry.hash & HASH22_MASK) >> shift;
+            if stored == target {
+                return Some(entry.callsign.clone());
+            }
+            idx = (idx + 1) % CALLSIGN_HASHTABLE_SIZE;
+            if idx == start_idx {
+                return None;
             }
         }
     }

src/decoders/trx-ftx/src/common/text.rs

@@ -64,10 +64,8 @@ pub fn charn(mut c: i32, table: CharTable) -> char {
                 return EXTRAS[c as usize];
             }
         }
-        CharTable::AlphanumSpaceSlash => {
-            if c == 0 {
-                return '/';
-            }
+        CharTable::AlphanumSpaceSlash if c == 0 => {
+            return '/';
         }
         _ => {}
     }
@@ -116,10 +114,8 @@ pub fn nchar(c: char, table: CharTable) -> Option<i32> {
             '?' => return Some(n + 4),
             _ => {}
         },
-        CharTable::AlphanumSpaceSlash => {
-            if c == '/' {
-                return Some(n);
-            }
+        CharTable::AlphanumSpaceSlash if c == '/' => {
+            return Some(n);
         }
         _ => {}
     }