Input: fix the example of an input device driver

This patch fixes a wrong interrupt handler example given in the "Hello,
world!"-like input driver in Documentation/input/input-programming.txt.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>