USB: gadget: Hangup tty on g_serial disconnect

On USB cable disconnect g_serial doesn't hangup the port tty,
which results in an endless read on the tty device. With the
following patch the read and select behave correctly when
the cable is unplugged.

Tested on at91rm9200

Signed-off-by: Savin Zlobec <savin@epiko.si>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>