USB: serial: ark3116: clean up return values of register accessors

write_reg returns 0 on success, we can make it more explicit by returning
number 0 instead of result variable.

read_reg should return 0 on success since this is a more common pattern.

The user of read_reg has been clean-up and should be at the same commit.

Signed-off-by: Gimcuan Hui <gimcuan@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>