MAINTAINERS: Use linux-usb mailing list for Thunderbolt and USB4 patches

MAINTAINERS: Use linux-usb mailing list for Thunderbolt and USB4 patches

Now that Thunderbolt public specification is called USB4 and is coming
from USB IF it makes sense to use linux-usb as mailing list for patches
touching this driver.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20200108125317.36444-3-mika.westerberg@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>