staging:iio:hmc5843: Rename _check_samp_freq to get_samp_freq_index

and drop/inline helper functions _check_int_plus_micros() and
_show_int_plus_micros()

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>