HID: core: do not scan reports if the group is already set

This allows the transport layer (I have in mind hid-logitech-dj and uhid)
to set the group before it is added to the hid bus. This way, it can
bypass the hid_scan_report() call, and choose in advance which driver
will handle the newly created hid device.

Signed-off-by: Benjamin Tisssoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>