drivers/net/skfp: fix sparse warnings: make symbols static

Fix this sparse warnings:

  drivers/net/skfp/skfddi.c:620:13: warning: symbol 'skfp_interrupt' was not declared. Should it be static?
  drivers/net/skfp/skfddi.c:687:25: warning: symbol 'skfp_ctl_get_stats' was not declared. Should it be static?
  drivers/net/skfp/skfddi.c:1232:6: warning: symbol 'CheckSourceAddress' was not declared. Should it be static?

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