MAINTAINERS: update web URL for iwlwifi

The current URL mentioned in iwlwifi's W entry is outdated and
currently pointing to a dead link.  Change it so that it points to the
correct Wiki page directly.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>