rtw89: ser: leave lps with mutex

Calling rtw89_leave_lps() should hold rtwdev::mutex.
So, fix it.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220704023453.19935-5-pkshih@realtek.com