Move prototype declaration of function to header file oss/pas2.h
from oss/pas2_card.c because it is used by more than one file.

This eliminates the following warnings in oss/pas2_mixer.c:
sound/oss/pas2_mixer.c:62:1: warning: no previous prototype for ‘mix_write’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>