Fix uninitialized return value from rpmsg callback

The variable used for the return value from the rpmsg callback to handle
mailbox messages isn't initialized when handling some message types.

To resolve this, the variable is now initialized by default.

Change-Id: I7862a6bc3479f03ca06536d94eb4478263e49b70
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>