Staging: serqt_usb2: add missing calls to tty_kref_put()

tty_port_tty_get() was called without a corresponding tty_kref_put()
in qt_read_bulk_callback() and qt_close().

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>