cpmac: fix the receiving of 802.1q frames

Despite what the comment above CPMAC_SKB_SIZE says, the hardware also
needs to account for the FCS length in a received frame. This patch fix
the receiving of 802.1q frames which have 4 more bytes. While at it
unhardcode the definition and use the one from if_vlan.h.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>