rtw88: enclose c2h cmd handle with mutex

C2H commands that cannot be handled in IRQ context should
be protected by rtwdev->mutex. Because they might have a
sequece of hardware operations that does not want to be
interfered.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>