iio:humidity:si7020: cleanup read_raw and probe

When reading temperature or humidity values, a shift of two bits to the right
needs to be applied, and only for the humidity channel a mask of the lower
12 bits needs to be applied. This reduces code repetition.
During probe, i2c_set_clientdata() was used, although its counterpart was not,
so drop it.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>