68328serial: remove unsed m68k_serial->tqueue_hangup

m68k_serial->tqueue_hangup is unused.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>