sound: Move prototype declaration to header file oss/pas2.h from oss/pas2_card.c

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>

sound/oss/pas2.h

@@ -15,3 +15,6 @@ int pas_init_mixer(void);
 /*	From pas_midi.c */
 void pas_midi_init(void);
 void pas_midi_interrupt(void);
+
+/*	From pas2_mixer.c*/
+void mix_write(unsigned char data, int ioaddr);

sound/oss/pas2_card.c

@@ -74,8 +74,6 @@ static char    *pas_model_names[] = {
  * to support other than the default base address
  */
 
-extern void     mix_write(unsigned char data, int ioaddr);
-
 unsigned char pas_read(int ioaddr)
 {
 	return inb(ioaddr + pas_translate_code);