iio: adf4350: fix compiler warning [-Wuninitialized]

drivers/iio/frequency/adf4350.c:316:32: warning: ‘val’ may be used uninitialized in this function
[-Wuninitialized]

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>