MAINTAINERS: mark prism54 obsolete

The prism54 driver had an entry in feature-removal-schedule.txt and it
sees very little activity other than API-change "bombing runs".  The
mac80211-based p54 driver should be used instead.
Signed-off-by: John W. Linville <linville@tuxdriver.com>

MAINTAINERS

@@ -4499,7 +4499,7 @@ PRISM54 WIRELESS DRIVER
 M:	"Luis R. Rodriguez" <mcgrof@gmail.com>
 L:	linux-wireless@vger.kernel.org
 W:	http://prism54.org
-S:	Maintained
+S:	Obsolete
 F:	drivers/net/wireless/prism54/
 
 PROMISE DC4030 CACHING DISK CONTROLLER DRIVER