HID: logitech-dj: fix variable naming in logi_dj_hidpp_event

we are not dealing with a dj_report but a hidpp_event.
We don't need all of the struct description in this function, but having
the variable named `dj_report` feels weird.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>