staging: iio: tsl2x7x: cleaned up i2c calls in tsl2x7x_als_calibrate()

The calibration function calls i2c_smbus_write_byte() and
i2c_smbus_read_byte(). These two function calls are replaced with a
single call to i2c_smbus_read_byte_data() by this patch. This patch
also removes an unnecessary call that reads the CNTRL register
a second time. One of the error paths returned -1 if the ADC was not
enabled and this patch changes that return value to -EINVAL.

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