Include the corner cases in the computation of the MIXER_VGATE register
value.

According to the datasheet: The MIXER_VGATE values follows the VCM such
as, that for a 0V to 1.8V VCM, MIXER_VGATE = 23.89 VCM + 81, and for a >
1.8V to 2.6V VCM, MIXER_VGATE = 23.75 VCM + 1.25.

Fixes: da35a7b5 ("iio: frequency: admv1013: add support for ADMV1013")
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230807143806.6954-1-antoniu.miclaus@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>