staging: iio: tsl2583: add locking to sysfs attributes

in_illuminance_input_target_show(), in_illuminance_input_target_store(),
in_illuminance_calibrate_store(), and in_illuminance_lux_table_store()
accesses data from the tsl2583_chip struct. Some of these fields can be
modified by other parts of the driver concurrently. This patch adds the
mutex locking to these sysfs attributes.

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