iio:proximity:sx9324: Fix hardware gain read/write

iio:proximity:sx9324: Fix hardware gain read/write

There are four possible gain values according to 'sx9324_gain_vals[]':

	1, 2, 4, and 8

The values are off by one when writing and reading the register. The
bits should be set according to this equation:

	ilog2(<gain>) + 1

so that a gain of 8 is 0x4 in the register field and a gain of 4 is 0x3
in the register field, etc. Note that a gain of 0 is reserved per the
datasheet. The default gain (SX9324_REG_PROX_CTRL0_GAIN_1) is also
wrong. It should be 0x1 << 3, i.e. 0x8, not 0x80 which is setting the
reserved bit 7.

Fix this all up to properly handle the hardware gain and return errors
for invalid settings.

Fixes: 4c18a890dff8 ("iio:proximity:sx9324: Add SX9324 support")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20220324222928.874522-1-swboyd@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>