MAINTAINERS: extend firmware_class maintainer list

I've been reviewing changes proactively, and plan on doing
more of this work. I'm doing this early as I should be getting
e-mailed about proposed changes.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>