rtlwifi: fix -Wpointer-sign warning

There are thousands of warnings in a W=2 build from just one file:

drivers/net/wireless/realtek/rtlwifi/rtl8821ae/table.c:3788:15: warning: pointer targets in initialization of 'u8 *' {aka 'unsigned char *'} from 'char *' differ in signedness [-Wpointer-sign]

Change the types to consistently use 'const char *' for the
strings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201026213040.3889546-6-arnd@kernel.org