rtw88: add a debugfs entry to enable/disable coex mechanism

Sometimes we need to stop the coex mechanism to debug, so that we
can manually control the device through various outer commands.
Hence, add a new debugfs coex_enable to allow us to enable/disable
the coex mechanism when driver is running.

To disable coex
echo 0 > /sys/kernel/debug/ieee80211/phyX/rtw88/coex_enable

To enable coex
echo 1 > /sys/kernel/debug/ieee80211/phyX/rtw88/coex_enable

To check coex dm is enabled or not
cat /sys/kernel/debug/ieee80211/phyX/rtw88/coex_enable

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200313033008.20070-3-yhchuang@realtek.com