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

Fix this sparse warnings:

  drivers/net/arcnet/capmode.c:64:6: warning: symbol 'arcnet_cap_init' was not declared. Should it be static?
  drivers/net/arcnet/com90xx.c:586:5: warning: symbol 'com90xx_reset' was not declared. Should it be static?

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