staging: iio: adt7316: change interpretation of write to dac update mode

Based on the output of adt7316_show_all_DAC_update_modes() and
adt7316_show_DAC_update_mode(), adt7316_store_DAC_update_mode() should
expect the user to enter an integer input from 0 to 3. The user input is
currently expected to account for the actual bit positions in the register.
For example, choosing option 3 would require a write of 0x30 (actually 48
since it expects base 10). To address this inconsistency, create a shift
macro to be used in the valid input check as well as the calculation for
the register write.

Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>