pcm: file: use snd_pcm_file_areas_read_infile for readi

pcm: file: use snd_pcm_file_areas_read_infile for readi

use previously introduced helper function, this commit unifies behavior
of readi and readn

corner case behavior of readi is changed by this commit, previously,
in case 0 bytes were red from file (EOF), frames = 0 was returned,
signaling api user as if no data was red from slave, after the patch,
amount of frames red from slave with data red from slave stored in buffer
is returned when EOF is reached

Signed-off-by: Adam Miartus <amiartus@de.adit-jv.com>
Reviewed-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>