iio: ad5755: fix off-by-one on devnr limit check

The comparison for devnr limits is off-by-one, the current check
allows 0 to AD5755_NUM_CHANNELS and the limit should be in fact
0 to AD5755_NUM_CHANNELS - 1.  This can lead to an out of bounds
write to pdata->dac[devnr]. Fix this by replacing > with >= on the
comparison.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Fixes: c9474599 ("iio: ad5755: add support for dt bindings")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>