firmware: dmi_scan: Make dmi_walk and dmi_walk_early return real error codes

Currently they return -1 on error, which will confuse callers if
they try to interpret it as a normal negative error code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Signed-off-by: Jean Delvare <jdelvare@suse.de>