MAINTAINERS: add myself as co-maintainer of the iwlwifi driver

I'm starting to take a more active role in the iwlwifi driver
maintainership.

Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>