drivers/net/wireless/prism54: fix sparse warning: fix signedness

Fix sparse warning:
  drivers/net/wireless/prism54/isl_ioctl.c:2997:32: warning: incorrect type in argument 2 (different signedness)
  drivers/net/wireless/prism54/oid_mgt.c:712:42: 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>