staging: iio: isl29028: move failure logging into isl29028_set_proxim_sampling()

When isl29028_set_proxim_sampling() fails, it was up to both callers to
log the failure message. This patch moves the logging into
isl29028_set_proxim_sampling() to reduce the overall amount of code in
the driver.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>