iwlwifi: make IWLWIFI a tristate

IWLWIFI should be a tristate so that if IWLCORE and/or IWL3945 are m
and none of them is y kbuild doesn't create an empty 
drivers/net/wireless/built-in.o

This patch also removes the pointless "default n".
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wireless/iwlwifi/Kconfig

 config IWLWIFI
-	bool
-	default n
+	tristate
 
 config IWLCORE
 	tristate "Intel Wireless Wifi Core"