ar5523: make buffer size variable unsigned

A negative buffer size doesn't make sense and it breaks this check in
ar5523_get_max_rxsz():

	if (!ar->rxbufsz || ar->rxbufsz > AR5523_SANE_RXBUFSZ) { ...
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ar5523/ar5523.h

@@ -122,7 +122,7 @@ struct ar5523 {
 
 	struct work_struct	rx_refill_work;
 
-	int			rxbufsz;
+	unsigned int		rxbufsz;
 	u8			serial[16];
 
 	struct ieee80211_channel channels[14];