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>
diff --git a/include/linux/iio/frequency/adf4350.h b/include/linux/iio/frequency/adf4350.h
index b76b4a8..be91f34 100644
--- a/include/linux/iio/frequency/adf4350.h
+++ b/include/linux/iio/frequency/adf4350.h
@@ -87,6 +87,8 @@
 #define ADF4350_MAX_BANDSEL_CLK		125000 /* Hz */
 #define ADF4350_MAX_FREQ_REFIN		250000000 /* Hz */
 #define ADF4350_MAX_MODULUS		4095
+#define ADF4350_MAX_R_CNT		1023
+
 
 /**
  * struct adf4350_platform_data - platform specific information