Input: synaptics - fix crash when enabling pass-through port

When enabling a pass-through port an interrupt might come before psmouse
driver binds to the pass-through port. However synaptics sub-driver
tries to access psmouse instance presumably associated with the
pass-through port to figure out if only 1 byte of response or entire
protocol packet needs to be forwarded to the pass-through port and may
crash if psmouse instance has not been attached to the port yet.

Fix the crash by introducing open() and close() methods for the port and
check if the port is open before trying to access psmouse instance.
Because psmouse calls serio_open() only after attaching psmouse instance
to serio port instance this prevents the potential crash.

Reported-by: Takashi Iwai <tiwai@suse.de>
Fixes: 100e1695 ("Input: libps2 - attach ps2dev instances as serio port's drvdata")
Link: https://bugzilla.suse.com/show_bug.cgi?id=1219522


Cc: stable@vger.kernel.org
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/Z4qSHORvPn7EU2j1@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>