wifi: rtw89: 8851b: Modify rtw8851b_pwr_{on,off}_func() for USB

There are a few differences in the power on/off functions between PCIE
and USB.

While the RTL8851BU appears to work without these changes, it's
probably best to implement them, in case they are needed in some
situations.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/469f6882-1859-464d-8a84-9ef1b6c8ce3e@gmail.com