drivers/net/ns83820.c: fix sparse warnings: fix signedness

Fix this sparse warnings:
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)
  drivers/net/ns83820.c:479:36: warning: incorrect type in argument 2 (different signedness)

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