wifi: rtw88: usb: Init RX burst length according to USB speed

This is needed in order to make USB RX aggregation work with RTL8811CU
(and presumably RTL8822BU and RTL8822CU also).

I don't know what BIT_DMA_BURST_CNT, BIT_DMA_MODE, and BIT_DROP_DATA_EN
are doing.

Tested with RTL8822CU, RTL8811CU, and RTL8723DU.

The RX speed is unchanged in my tests.

Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/ac569c6f-7129-4341-b523-901fe10cabff@gmail.com