Make snd_pcm_lib_read() and *_write() static inline functions that
call the common helper functions directly.  This reduces a slight
amount of codes, and at the same time, it's a preparation for the
further cleanups / fixes.
Reviewed-by: NTakashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>