wifi: ath6kl: reduce WARN to dev_dbg() in callback

The warn is triggered on a known race condition, documented in the code above
the test, that is correctly handled.  Using WARN() hinders automated testing.
Reducing severity.

Fixes: de2070fc

 ("ath6kl: Fix kernel panic on continuous driver load/unload")
Reported-and-tested-by:  <syzbot+555908813b2ea35dae9a@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230126182431.867984-1-pchelkin@ispras.ru