staging: iio: tsl2x7x: add error handling to tsl2x7x_prox_cal()

tsl2x7x_prox_cal() did not have any error checks. This patch adds
the missing error handling and ensures that any errors are reported to
user space via in_proximity0_calibrate_store().

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>