max44000: Remove scale from proximity

This is not implemented and doesn't really make sense because IIO
proximity is unit-less.

Remove IIO_CHAN_INFO_SCALE from info_mask because so that the _scale
sysfs entry won't appear. This fixes userspace tools like generic_buffer
which abort when reads returns an error.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>