WDEV: ath5k, fix lock imbalance

Omitted lock causes sparse warning
drivers/net/wireless/ath5k/base.c:1682:1: warning: context imbalance in 'ath5k_tasklet_rx' - different lock contexts for basic block

Add the lock to the guilty fail path.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath5k/base.c

@@ -1715,6 +1715,7 @@ ath5k_tasklet_rx(unsigned long data)
 			break;
 		else if (unlikely(ret)) {
 			ATH5K_ERR(sc, "error in processing rx descriptor\n");
+			spin_unlock(&sc->rxbuflock);
 			return;
 		}