iio: frequency: adf4350: using an uninitialized variable

GCC complains that we use an uninitialized variable if the user passes
an invalid parameter to adf4350_read().  I decided that we should return
-EINVAL instead in that case.

However, when I looked up at adf4350_write() it returned -ENODEV for
that condition.  In the end, I decided the -EINVAL was the right thing
and I change adf4350_write() to match.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>