wifi: rtl8xxxu: Make rtl8xxxu_load_firmware take const char*

And pass const char* to it.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/361ceac1-cc73-605b-4b63-736bfce80833@gmail.com