iio: adc: max11100: Use get_unaligned_be16() rather than opencoding.

The function is more explicit in showing the intent + quicker on some
platforms.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210516172520.1398835-2-jic23@kernel.org