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>