mailbox: mailbox-test: Use more consistent format for calling copy_from_user()

While we're at it, ensure copy-to location is NULL'ed in the error path.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>