[IPV4]: Fix thinko in TCP_CONG_BIC default.

Since it is tristate when we offer it as a choice, we should
definte it also as tristate when forcing it as the default.
Otherwise kconfig warns.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 3470834..7342a9a 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -549,7 +549,7 @@
 endmenu
 
 config TCP_CONG_BIC
-	boolean
+	tristate
 	depends on !TCP_CONG_ADVANCED
 	default y