libertas: add Dan Williams as maintainer

Add Dan Williams as maintainer for libertas driver.
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

MAINTAINERS

@@ -2489,6 +2489,12 @@ M:	mtk.manpages@gmail.com
 W:	ftp://ftp.kernel.org/pub/linux/docs/manpages
 S:	Maintained
 
+MARVELL LIBERTAS WIRELESS DRIVER
+P:	Dan Williams
+M:	dcbw@redhat.com
+L:	libertas-dev@lists.infradead.org
+S:	Maintained
+
 MARVELL MV643XX ETHERNET DRIVER
 P:	Dale Farnsworth
 M:	dale@farnsworth.org