drivers/net/enic: fix sparse warning: make symbol static

Fix this sparse warning:

  drivers/net/enic/vnic_dev.c:288:5: warning: symbol 'vnic_dev_capable' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>