rtw88: 8821c: coex: add functions and parameters

Without this patch, RTL8821CE will not have coex support,
and will crash the system because of the NULL pointers
for the coex functions.

While RTL8822C series are WiFi + BT combo chips, it needs
the co-existence mechanism for the device to work on both
WiFi and BT without interfering each other. And the coex
support has already been added before, most of the mechanisms
are implemented. The driver should just add corresponding
functions to operate on different types of chips and its
coex parameters.

Fixes: f745eb9c ("rtw88: 8821c: Add 8821CE to Kconfig and Makefile")
Signed-off-by: Ping-Cheng Chen <pc.chen@realtek.com>
Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200724054208.31115-1-yhchuang@realtek.com