libcxgbi/cxgb4i : Fix ipv6 build failure caught with randconfig

Previous guard of IS_ENABLED(CONFIG_IPV6) is not sufficient when cxgbi drivers
are built into kernel but ipv6 is not.

v2: Use Kconfig to disable compiling cxgbi built into kernel when ipv6 is
compiled as a module

Fixes: e81fbf6c ("libcxgbi:cxgb4i Guard ipv6 code with a config check")
Fixes: fc8d0590 ("libcxgbi: Add ipv6 api to driver")
Signed-off-by: NAnish Bhatt <anish@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/scsi/cxgbi/cxgb3i/Kconfig

 config SCSI_CXGB3_ISCSI
 	tristate "Chelsio T3 iSCSI support"
-	depends on PCI && INET
+	depends on PCI && INET && (IPV6 || IPV6=n)
 	select NETDEVICES
 	select ETHERNET
 	select NET_VENDOR_CHELSIO

drivers/scsi/cxgbi/cxgb4i/Kconfig

 config SCSI_CXGB4_ISCSI
 	tristate "Chelsio T4 iSCSI support"
-	depends on PCI && INET
+	depends on PCI && INET && (IPV6 || IPV6=n)
 	select NETDEVICES
 	select ETHERNET
 	select NET_VENDOR_CHELSIO