r8152: Remove unused function usb_ocp_read()

The function is not used, removing it fixes the following warning when
building with clang:

drivers/net/usb/r8152.c:825:5: error: unused function 'usb_ocp_read'
    [-Werror,-Wunused-function]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>