staging:iio:adt7410: Fix adt7410_set_mode return value

The function is expected to return the number of bytes consumed and as long as
not all bytes have been consumed the function will be called again. Currently
the function returns 'ret', which will always be 0 in this case, so we end up in
a endless loop since the caller will assume that no bytes have been consumed. So
instead return len as it is supposed to.

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