iio:accel:mma8452: fix _get_hp_filter_index

To iterate through the available frequencies of mma8452_hp_filter_cutoff[],
the array size of a row of that table needs to be provided to
_get_int_plus_micros_index().

Fixes: 	1e79841a ("iio: mma8452: Add highpass filter configuration.")

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>