drivers/net/wireless/ipw2x00: fix sparse warnings: fix signedness

Fix this sparse warnings:
  drivers/net/wireless/ipw2x00/ipw2100.c:1930:43: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:1938:76: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:1946:67: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:1953:72: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:4071:73: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:4078:43: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:4084:67: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:7141:73: warning: incorrect type in argument 4 (different signedness)
  drivers/net/wireless/ipw2x00/ipw2100.c:8317:48: warning: incorrect type in argument 4 (different signedness)

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