drivers/iio/adc/ad9361: Fix Slow Attack AGC default config for ClkRF below 8 MSPS

Make sure that there are at least two power measurements during each
gain update interval. Use user specified decimated power measurement
duration, however do sanity check and decrease if necessary.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Merge branch 'xcomm_zynq' of https://github.com/analogdevicesinc/linux into xcomm_zynq