iio: frequency: adf4371: Fix output frequency setting

The fract1 word was not being properly programmed on the device leading
to wrong output frequencies.

Fixes: 7f699bd1

 (iio: frequency: adf4371: Add support for ADF4371 PLL)
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Stefan Popa <stefan.popa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>