[mle] simplify condition in Mle::BecomeDetached() (openthread#10796)

This commit updates the skip condition in `BecomeDetached()` to use
positive boolean checks for improved readability. This condition
applies when the device is already detached and is about to start an
attach attempt.

src/core/thread/mle.cpp

@@ -501,11 +501,12 @@ Error Mle::BecomeDetached(void)
 
     VerifyOrExit(!IsDisabled(), error = kErrorInvalidState);
 
-    // In case role is already detached and attach state is `kAttachStateStart`
-    // (i.e., waiting to start an attach attempt), there is no need to make any
-    // changes.
-
-    VerifyOrExit(!IsDetached() || mAttachState != kAttachStateStart);
+    if (IsDetached() && (mAttachState == kAttachStateStart))
+    {
+        // Already detached and waiting to start an attach attempt, so
+        // there is not need to make any changes.
+        ExitNow();
+    }
 
     // Not in reattach stage after reset
     if (mReattachState == kReattachStop)