Fix RDAC read back errors caused by a typo. Value must shift by 2.

Fixes: a4bd3949 ("drivers/misc/ad525x_dpot.c: new features")
Signed-off-by: NMichael Hennerich <michael.hennerich@analog.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>