drivers: mbox: Fix error code in Nordic's implementation

According to the official MBOX API documentation available at
https://docs.zephyrproject.org/latest/hardware/peripherals/mbox.html


mbox_sen() shall return
-EMSGSIZE – If the supplied data size is unsupported by the driver.

Fix error code returned by the mbox_send().

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>