Bluetooth: btusb: Ignore zero length of USB packets on ALT 6 for specific chip

For USB ALT 6 settings some Realtek chips need to transmit mSBC data
continuously without the zero length of USB packets.
In this commit, create BTUSB_ALT6_CONTINUOUS_TX to manage the behavior.
Therefore, create REALTEK_ALT6_CONTINUOUS_TX_CHIP to manage the specific
chip model for the behavior.

Signed-off-by: Max Chou <max.chou@realtek.com>
Signed-off-by: Hilda Wu <hildawu@realtek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>