staging: r8188eu: dvobj_priv's signal_strength is not used

signal_strength is struct dvobj_priv is not used. Remove it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220206175925.461426-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>