ALSA: hda - Cancel probe work instead of flush at remove

The commit [991f86d7: ALSA: hda - Flush the pending probe work at
remove] introduced the sync of async probe work at remove for fixing
the race.  However, this may lead to another hangup when the module
removal is performed quickly before starting the probe work, because
it issues flush_work() and it's blocked forever.

The workaround is to use cancel_work_sync() instead of flush_work()
there.

Fixes: 991f86d7

 ('ALSA: hda - Flush the pending probe work at remove')
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Takashi Iwai <tiwai@suse.de>