iio: temperature: max31856: fix uninitialized error return

Currently if mask is neither CHAN_INFO_RAW or CHAN_INFO_SCALE then
then an uninitialized error return 'ret' is returned. Fix this by
adding a default case that ensures -EINVAL is returned for this
specific case.

Addresses-Coverity: ("Uninitialized scalar variable")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>