rtw88: remove extraneous 'const' qualifier

clang -Wextra warns about functions returning a 'const' integer:

drivers/net/wireless/realtek/rtw88/rtw8822b.c:90:8: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
static const u8 rtw8822b_get_swing_index(struct rtw_dev *rtwdev)

Remove the extra qualifier here.

Fixes: c97ee3e0 ("rtw88: add power tracking support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201026213040.3889546-7-arnd@kernel.org