firmware: microchip: use scope-based cleanup where possible

There's a bunch of structs created and freed every time the mailbox is
used. Move them to use the scope-based cleanup infrastructure to avoid
manually tearing them down. mpfs_auto_update_available() didn't free the
memory that it used (albeit it allocated exactly once during probe) so
that gets moved over too.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>