HID: core: factor out hid_set_group()

HID: core: factor out hid_set_group()

When we load a bpf, we can change the report descriptor. However, the
current implementation doesn't change the group meaning that we can not
rebind a device from hid-generic to hid-multitouch.

This is a preparatory patch to force a rescan of the device after the
bpf has been loaded.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.com>