iio: frequency: ADF4350: Fix potential reference div factor overflow.

With small channel spacing values and high reference frequencies it is
possible to exceed the range of the 10-bit counter.
Workaround by checking the range and widening some constrains.

We don't use the REG1_PHASE value in this case the datasheet recommends to set
it to 1 if not used.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>