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>