iio: ad9523: Fix argument type mismatch

drivers/iio/frequency/ad9523.c:378 ad9523_vco_out_map()
	 warn: value 2 can't fit into 1 'out'

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>