HID: logitech-dj: rename dj_receiver_dev.hdev to dj_receiver_dev.hidpp

For the upcoming non-unifying receiver support, we are going to bind to
all USB-interfaces of a receiver, sharing a single struct dj_receiver_dev
between the interfaces. This means that dj_receiver_dev will contain
multiple pointers to a struct hid_device. Rename the current hdev member
to hidpp to prepare for this.

While at it switch dev_err calls which we are touching anyways from
dev_err to hid_err.

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