ALSA: line6: Fix the error recovery in line6_pcm_acquire()

line6_pcm_acquire() tries to restore the newly obtained resources at
the error path.  But some flags aren't recorded and released properly
when the corresponding buffer is already present.  These bits have to
be cleared in the error recovery, too.

Also, "flags_final" can be initialized to zero since we pass only the
subset of "channels" bits.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>