ALSA: pcm: Add __force to cast in snd_pcm_lib_read/write()

The snd_pcm_lib_read() and snd_pcm_lib_write() inline functions have
the explicit cast from a user pointer to a kernel pointer, but they
lacks of __force prefix.

This fixes sparse warnings like:
  ./include/sound/pcm.h:1093:47: warning: cast removes address space of expression

Fixes: 68541213 ("ALSA: pcm: Direct in-kernel read/write support")
Signed-off-by: Takashi Iwai <tiwai@suse.de>