iio: accel: mma8452: Rename read/write event value callbacks to generic function name.

'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@gmail.com>
Acked-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>