mailbox: Stop using ENOSYS for anything other than unimplemented syscalls

In accordance with e15f431f ("errno.h: Improve ENOSYS's comment") and
91c9afaf ("checkpatch.pl: new instances of ENOSYS are errors") we're
converting from the old meaning of: ENOSYS "Function not implemented" to
a more standard EINVAL.
Reported-by: NSeraphin Bonnaffe <seraphin.bonnaffe@st.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox.c

@@ -375,13 +375,13 @@ struct mbox_chan *mbox_request_channel_byname(struct mbox_client *cl,
 
 	if (!np) {
 		dev_err(cl->dev, "%s() currently only supports DT\n", __func__);
-		return ERR_PTR(-ENOSYS);
+		return ERR_PTR(-EINVAL);
 	}
 
 	if (!of_get_property(np, "mbox-names", NULL)) {
 		dev_err(cl->dev,
 			"%s() requires an \"mbox-names\" property\n", __func__);
-		return ERR_PTR(-ENOSYS);
+		return ERR_PTR(-EINVAL);
 	}
 
 	of_property_for_each_string(np, "mbox-names", prop, mbox_name) {