ALSA: us122l: Use alloc_pages_exact()

alloc_pages_exact() is more suitable choice for allocating the sound
buffers, as it doesn't need to align with power-of-two.  Along with
the conversion, we can drop __GFP_COMP as well.

The patch also replace the error messages to be more explicit.

Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>