V4L/DVB (13593): PWC: parameter trace is only available in debug

This patch fixes a small issue where modinfo says the parameter "trace" is always available, while
it only works if CONFIG_USB_PWC_DEBUG is enabled.

Signed-off-by: Andrea Odetti <mariofutire@googlemail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>