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>

include/linux/hdlcdrv.h

@@ -215,7 +215,7 @@ struct hdlcdrv_state {
 
 	struct hdlcdrv_hdlctx {
 		struct hdlcdrv_hdlcbuffer hbuf;
-		long in_hdlc_tx;
+		unsigned long in_hdlc_tx;
 		/*
 		 * 0 = send flags
 		 * 1 = send txtail (flags)