r8152: remove rtl_vendor_mode function

After commit ec51fbd1

 ("r8152: add USB device driver for
config selection"), the code about changing USB configuration
in rtl_vendor_mode() wouldn't be run anymore. Therefore, the
function could be removed.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>