r8169: add helper r8168d_modify_extpage

Certain integrated PHY's from RTL8168d support extended pages. On page
0x0007 the number of the extended page is written to register 0x1e,
then the registers on the extended page can be accessed. Add a helper
for this to improve readability and simplify the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>