rtw88: 8821c: make symbol 'rtw8821c_rtw_pwr_track_tbl' static

The sparse tool complains as follows:

drivers/net/wireless/realtek/rtw88/rtw8821c.c:1374:32: warning:
 symbol 'rtw8821c_rtw_pwr_track_tbl' was not declared. Should it be static?

This variable is not used outside of rtw8821c.c, so this commit
marks it static.

Fixes: 3a431282 ("rtw88: 8821c: add power tracking")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200721134901.33968-1-weiyongjun1@huawei.com