mailbox: mailbox-test: initialize struct earlier

mailbox: mailbox-test: initialize struct earlier

The waitqueue must be initialized before the debugfs files are created
because from that time, requests from userspace can already be made.
Similarily, drvdata and spinlock needs to be initialized before we
request the channel, otherwise dangling irqs might run into problems
like a NULL pointer exception.

Fixes: 8ea4484d0c2b ("mailbox: Add generic mechanism for testing Mailbox Controllers")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>