Input: gpio_keys - remove useless reinitialization of pdata->nbuttons

pdata is zeroed using memset just a few lines before, so there is no
need to set the nbuttons member to 0 again.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>