'mma8452_read_thresh' and 'mma8452_write_thresh' functions
does more than just read/write threshold values.
They also handle IIO_EV_INFO_HIGH_PASS_FILTER_3DB and
IIO_EV_INFO_PERIOD therefore renaming to generic names.
Improves code readability, no impact on functionality.
Signed-off-by: Harinath Nampally <harinath922@xxxxxxxxx>