iio: imu: kmx61: Drop odr_bits from kmx61_samp_freq_table

odr_bits values are between 0 and 11, so we can use the index
in kmx61_samp_freq_table instead of odr_bits structure member.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>