wlcore: fix runtime pm imbalance in wl1271_tx_work

There are two error handling paths in this functon. When
wlcore_tx_work_locked() returns an error code, we should
decrease the runtime PM usage counter the same way as the
error handling path beginning from pm_runtime_get_sync().

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200520124241.9931-1-dinghao.liu@zju.edu.cn