tty: USB tty devices can block in tcdrain when unplugged

The underlying problem is that the device methods don't all correctly
handle disconnected status and some keep reporting bytes pending which
causes tcdrain to stall.

When the cable is unplugged they are definitely gone, and as this is true
for all USB cables we can fix it in the core usb serial code.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>