Staging rtl8172: Remove unnecessary typecast

Using addressof and then casting to the original type is unneeded.
So these casts can be removed.
Issue detected via Coccinelle script.

Signed-off-by: Vatika Harlalka <vatikaharlalka@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>