r8169:Update the way of reading RTL8168H PHY register "rg_saw_cnt"

The vlaue of RTL8168H PHY register "rg_saw_cnt" only valid from bit0 to bit13.
When read this register, add bitwise-anding its value with 0x3fff.

Signed-off-by: Chunhao Lin <hau@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>