Input: wacom_i2c - do not use irq_to_gpio

Because irq_to_gpio() is not available on many platforms let's switch
to level-triggered one-shot IRQs that will stay active as long as
there is data to be read.

Tested-by: Tobita Tatsunosuke <tobita.tatsunosuke@wacom.co.jp>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>