r8169: Dereference MMIO address immediately before use

There is no need to dereference struct rtl8169_private to get mmio_addr
in almost every function in the driver.

Replace it by using pointer to struct rtl8169_private directly.

No functional change intended.

Next step might be a conversion of RTL_Wxx() / RTL_Rxx() macros
to inline functions for sake of type checking.

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>