HID: elo: update the reference count of the usb device structure

Use usb_get_dev() to increment the reference count of the usb device
structure in order to avoid releasing the structure while it is still in
use. And use usb_put_dev() to decrement the reference count and thus,
when it will be equal to 0 the structure will be released.

Signed-off-by: Salah Triki <salah.triki@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>