m68k: nfcon: Do not call console_is_registered() in nfcon_device()

Since 7c2af0f634f1 ("tty: tty_io: use console_list_lock for list
synchronization") show_cons_active() calls the .device() method under
the console_list_lock, but console_is_registered() tries to acquire
console_list_lock as well, causing a deadlock.  It should not be
necessary to check console_is_registered() here since the function
should not be called in the fist place when the console is not
registered.

Fixes: 7c2af0f634f1 ("tty: tty_io: use console_list_lock for list synchronization")
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patch.msgid.link/87ecfzsv6h.fsf@igel.home
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>