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>

drivers/mbox/mbox_nrf_bellboard_tx.c

@@ -23,7 +23,7 @@ static int bellboard_tx_send(const struct device *dev, uint32_t id, const struct
 	}
 
 	if (msg != NULL) {
-		return -ENOTSUP;
+		return -EMSGSIZE;
 	}
 
 	nrfy_bellboard_task_trigger(config->bellboard, nrf_bellboard_trigger_task_get(id));

drivers/mbox/mbox_nrf_vevif_event_tx.c

@@ -38,7 +38,7 @@ static int vevif_event_tx_send(const struct device *dev, uint32_t id, const stru
 	}
 
 	if (msg != NULL) {
-		return -ENOTSUP;
+		return -EMSGSIZE;
 	}
 
 	nrf_vpr_csr_vevif_events_trigger(BIT(id));

drivers/mbox/mbox_nrf_vevif_task_tx.c

@@ -38,7 +38,7 @@ static int vevif_task_tx_send(const struct device *dev, uint32_t id, const struc
 	}
 
 	if (msg != NULL) {
-		return -ENOTSUP;
+		return -EMSGSIZE;
 	}
 
 	nrfy_vpr_task_trigger(config->vpr, nrfy_vpr_trigger_task_get(id));