tg3: Enable PLL PD when CLKREQ disabled for 5717A0

PCIe PLL power down cannot be used if CLKREQ is enabled because data
corruption will occur.  If CLKREQ is disabled though, enabling PCIE P1
PLL power-down saves some power.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>