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

Fix this sparse warnings:
  drivers/net/wireless/airo.c:2102:21: warning: incorrect type in initializer (different signedness)
  drivers/net/wireless/airo.c:2126:21: warning: incorrect type in initializer (different signedness)
  drivers/net/wireless/airo.c:2167:21: warning: incorrect type in initializer (different signedness)
  drivers/net/wireless/airo.c:2191:21: warning: incorrect type in initializer (different signedness)

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