serport - avoid calling serio_interrupt or serio_write_wakeup on unregistered
port.  Also fix memory leak which could happen if serport was left unused by
moving serio allocation down to serport_ldisc_read.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Cc: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>