staging: iio: tsl2583: remove redudant i2c call in taos_als_calibrate()

taos_als_calibrate() queries the control register to determine if the
unit is powered on and has the ADC enabled. It then queries the same
register a second time to determine if the ADC reading is valid. This
patch removes the redundant i2c_smbus_read_byte_data() call.

Verified that the driver still functions correctly using a TSL2581
hooked up to a Raspberry Pi 2.

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