iio: magnetometer: mmc35240: fix available sampling frequencies

Fix the sampling frequencies according to the datasheet (page 8). The
datasheet specifies the following available frequencies for continuous
mode: 1.5 Hz, 13 Hz, 25 Hz, and 50 Hz.

Also fix comments about the ODR to comply with datasheet.

Signed-off-by: Teodora Baluta <teodora.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>