wifi: rtw88: Drop coex mutex

coex->mutex is used in rtw_coex_info_request() only. Most callers of this
function hold rtwdev->mutex already, except for one callsite in the
debugfs code. The debugfs code alone doesn't justify the extra lock, so
acquire rtwdev->mutex there as well and drop the now unnecessary
spinlock.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221202081224.2779981-6-s.hauer@pengutronix.de