rtw88: fix compile warning: [-Wignored-qualifiers]

Remove function return type const in rtw8821c_get_swing_index().
This is unused, and unnecessary to be const.

All warnings (new ones prefixed by >>):

>> drivers/net/wireless/realtek/rtw88/rtw8821c.c:71:8: warning: type
qualifiers ignored on function return type [-Wignored-qualifiers]
       71 | static const u8 rtw8821c_get_swing_index(struct rtw_dev *rtwdev)
          |        ^~~~~

Fixes: f745eb9c ("rtw88: 8821c: Add 8821CE to Kconfig and Makefile")
Reported-by: kernel test robot <lkp@intel.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/20200902053402.12839-1-yhchuang@realtek.com