iio: adc: max11410: fix incomplete vref buffer mask

iio: adc: max11410: fix incomplete vref buffer mask

VREFP bit was missing from channel configuration mask and VREFN bit was
included twice instead which fails to enable positive reference buffer when
requested by a channel. Channels that don't enable vrefp buffer were not
affected.

Fixes: a44ef7c46097 ("iio: adc: add max11410 adc driver")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Ibrahim Tilki <Ibrahim.Tilki@analog.com>
Link: https://lore.kernel.org/r/20221122114718.17557-1-Ibrahim.Tilki@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>