net/ncsi: fix ncsi_vlan_rx_{add,kill}_vid references

We get a new link error in allmodconfig kernels after ftgmac100
started using the ncsi helpers:

ERROR: "ncsi_vlan_rx_kill_vid" [drivers/net/ethernet/faraday/ftgmac100.ko] undefined!
ERROR: "ncsi_vlan_rx_add_vid" [drivers/net/ethernet/faraday/ftgmac100.ko] undefined!

Related to that, we get another error when CONFIG_NET_NCSI is disabled:

drivers/net/ethernet/faraday/ftgmac100.c:1626:25: error: 'ncsi_vlan_rx_add_vid' undeclared here (not in a function); did you mean 'ncsi_start_dev'?
drivers/net/ethernet/faraday/ftgmac100.c:1627:26: error: 'ncsi_vlan_rx_kill_vid' undeclared here (not in a function); did you mean 'ncsi_vlan_rx_add_vid'?

This fixes both problems at once, using a 'static inline' stub helper
for the disabled case, and exporting the functions when they are present.

Fixes: 51564585 ("ftgmac100: Support NCSI VLAN filtering when available")
Fixes: 21acf630

 ("net/ncsi: Configure VLAN tag filter")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>