wifi: rtw89: fw: generalize download firmware flow by mac_gen pointers

In order to reuse the flow to download firmware, define some mac_gen::ops
to implement them for WiFi 6 and 7 chips individually. This doesn't change
logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230901073956.54203-3-pkshih@realtek.com