ALSA: firewire-lib/bebob/oxfw: improve response evaluation for AV/C commands

In ALSA firewire stack, some AV/C commands are supported, including
vendor's extensions. Drivers includes response parser of each command,
according to its requirements, while the parser is written with loose
fashion in two points; error check and length check. This doesn't cause
any issues such as kernel corruption, but should be improved.

This commit modifies evaluations of return value on each parsers.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>