Update iatomic.h functions definitions for mips

Update iatomic.h functions definitions for mips

Functions atomic_add(s) and atomic_sub(s) are defined with 'extern
__inline__' that may cause compile fails when cross compile for mips.
The error message looks like:

| pcm/.libs/libpcm.a(pcm_meter.o): In function `snd_pcm_meter_update_scope':
| .../alsa-lib-1.0.27.2/src/pcm/pcm_meter.c:139: undefined reference to `atomic_sub'

Replace the 'extern __inline__' with 'static __inline__' to fix this
issue.

Signed-off-by: Kai Kang <jiashuo.kang at gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>