cnic : Cleanup CONFIG_IPV6 & VLAN check

The cnic module needs to ensure that if ipv6 support is compiled as a module,
then the cnic module cannot be compiled as built-in as it depends on ipv6.
Made this check cleaner via Kconfig

Use simpler IS_ENABLED for CONFIG_VLAN_8021Q check

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>