mailbox: handle failed named mailbox channel request

mailbox: handle failed named mailbox channel request

Previously, if mbox_request_channel_byname was used with a name
which did not exist in the "mbox-names" property of a mailbox
client, the mailbox corresponding to the last entry in the
"mbox-names" list would be incorrectly selected.
With this patch, -EINVAL is returned if the named mailbox is
not found.

Signed-off-by: Morten Borup Petersen <morten_bp@live.dk>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>