Staging: wlan-ng: fix build error if wireless networking is not enabled

Thanks to Ingo for pointing this out.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/wlan-ng/Kconfig

 config PRISM2_USB
 	tristate "Prism2.5 USB driver"
-	depends on USB
+	depends on WLAN_80211 && USB
 	default n
 	---help---
 	  This is the wlan-ng prism 2.5 USB driver for a wide range of