ALSA: remove unnecessary null check

This function is only called from snd_ctl_ioctl() and the file parameter
can never be null so there is no need to check it here.

We dereference file at the start of the function:
        struct snd_card *card = file->card;
and it confuses static checkers to dereference a pointer before
checking it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>