iio: Always compute masklength

Even if no userspace consumer buffer is attached to the IIO device at
registration we still need to compute the masklength, since it is possible
that a in-kernel consumer buffer is going to get attached to the device at
a later point.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>