drivers: call adapter_get_required_serial() in jtag_libusb_open()

drivers: call adapter_get_required_serial() in jtag_libusb_open()

Now that adapter serial is handled independently from the adapter
drivers, move inside jtag_libusb_open() the call to
adapter_get_required_serial(), so every adapter that uses libusb
will automagically get USB serial support.

Extend the documentation to list the adapters involved.

Change-Id: I75b3482d38f8ed3418329f3106c5e8b689fd460b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6663
Tested-by: jenkins