wifi: rtw89: move enable_cpu/disable_cpu into fw_download

For WoWLAN mode, we need to download WoWLAN firmware by calling
fw_download(). Another, to disable/enable WiFi CPU is needed before
calling fw_download. Since Firmware runs on WiFi CPU, it is intuitive
to combine enable_cpu/disable_cpu functions into fw_download.

Signed-off-by: Chih-Kang Chang <gary.chang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221027052707.14605-3-pkshih@realtek.com