iio: frequency: adf4350: Fix prescaler usage.

The ADF4350/1 features a programmable dual-modulus prescaler of 4/5 or 8/9.
When set to 4/5, the maximum RF frequency allowed is 3 GHz.
Therefore, when operating the ADF4351 above 3 GHz, this must be set to 8/9.
In this context not the RF output frequency is meant
- it's the VCO frequency.

Therefore move the prescaler selection after we derived the VCO frequency
from the desired RF output frequency.

This BUG may have caused PLL lock instabilities when operating the VCO at
the very high range close to 4.4 GHz.

Fixes: e31166f0fd48 ("iio: frequency: New driver for Analog Devices ADF4350/ADF4351 Wideband Synthesizers")
Signed-off-by: Michael Hennerich <[e-mail]>
Signed-off-by: Nuno Sá <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/frequency/adf4350.c

···
 	if (freq > ADF4350_MAX_OUT_FREQ || freq < st->min_out_freq)
 		return -EINVAL;
 
+	st->r4_rf_div_sel = 0;
+
+	/*
+	 * !\TODO: The below computation is making sure we get a power of 2
+	 * shift (st->r4_rf_div_sel) so that freq becomes higher or equal to
+	 * ADF4350_MIN_VCO_FREQ. This might be simplified with fls()/fls_long()
+	 * and friends.
+	 */
+	while (freq < ADF4350_MIN_VCO_FREQ) {
+		freq <<= 1;
+		st->r4_rf_div_sel++;
+	}
+
 	if (freq > ADF4350_MAX_FREQ_45_PRESC) {
 		prescaler = ADF4350_REG1_PRESCALER;
 		mdiv = 75;
 	} else {
 		prescaler = 0;
 		mdiv = 23;
-	}
-
-	st->r4_rf_div_sel = 0;
-
-	while (freq < ADF4350_MIN_VCO_FREQ) {
-		freq <<= 1;
-		st->r4_rf_div_sel++;
 	}
 
 	/*