mailbox: mailbox-test: Fix an error check in mbox_test_probe()

mbox_test_request_channel() function returns NULL or
error value embedded in the pointer (PTR_ERR).
Evaluate the return value using IS_ERR_OR_NULL.

Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>