iio: dac: ad5770r: fix off-by-one check on maximum number of channels

Currently there is an off-by-one check on the number of channels that
will cause an arry overrun in array st->output_mode when calling the
function d5770r_store_output_range. Fix this by using >= rather than >
to check for maximum number of channels.

Addresses-Coverity: ("Out-of-bounds access")
Fixes: cbbb8198 ("iio: dac: ad5770r: Add AD5770R support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>