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

Fix this sparse warnings:
  drivers/net/hamradio/hdlcdrv.c:274:34: warning: incorrect type in argument 2 (different signedness)
  drivers/net/hamradio/hdlcdrv.c:279:47: warning: incorrect type in argument 2 (different signedness)
  drivers/net/hamradio/hdlcdrv.c:288:39: warning: incorrect type in argument 2 (different signedness)
  drivers/net/hamradio/hdlcdrv.c:300:47: 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>