Input: gameport - provide default trigger() and read()

Instead of constantly checking pointer(s) for non-NULL-ness provide
default implementations of trigger() and read() and instantiate them during
pore registration if driver-specific versions were not provided.

Link: https://lore.kernel.org/r/ZGvoqP5PAAsJuky4@google.com


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