net: usb: r8152: fix "duplicate 'const' declaration specifier"

After upgrading to GCC 7.1, the duplicate const specifies in the
r8152 driver trigger the following build warnings with buildman
(observed on a 'buildman rockchip' test)::
  ../drivers/usb/eth/r8152.c:62:35: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
   static const struct r8152_version const r8152_versions[] = {
                                     ^~~~~

This commit fixes these by removing the duplicate 'const' specifier
from the declarations.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>