rtlwifi: move spin_lock_bh to spin_lock in tasklet

It is unnecessary to call spin_lock_bh in a tasklet.

Signed-off-by: Jeff Xie <chongguiguzi@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>