iio: light: bh1780: return after write

When writing a value using direct reg access from debugfs
we need to return and not fall through to reading the
value, lest we'll dereference a NULL pointer.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>