net: rtl8169: Fix compile warning in rtl8169

While compiling rtl8169.c, There are many "make pointer from
integer without a cast" compile warnings. fix them with
adding cast.

Signed-off-by: Minda Chen <minda.chen@starfivetech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>