media: pci: mgb4: include linux/errno.h

The errno.h header is not always included indirectly, leading
to rare randconfig build warnings.

drivers/media/pci/mgb4/mgb4_regs.c:20:11: error: use of undeclared identifier 'EINVAL'
   20 |                 return -EINVAL;

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>