V4L/DVB (10242): pwc: add support for webcam snapshot button

This patch adds support for Philips webcam snapshot button as an
event input device, for consistency with other webcam drivers.

Signed-off-by: Pham Thanh Nam <phamthanhnam.ptn@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>