pch_gbe: Fix PTP dependencies.

The config combination:

        CONFIG_PCH_GBE=y
        CONFIG_PCH_PTP=y
        CONFIG_PTP_1588_CLOCK=m

doesn't work, because then you have a built-in kernel
object (the PCH_PTP code) referring to symbols in a
module (PTP_1588_CLOCK).

Fix this like IXGBE, by using "select PTP_1588_CLOCK"
instead of a "depends on".
Reported-by: Haicheng Li <haicheng.li@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/oki-semi/pch_gbe/Kconfig

@@ -26,7 +26,7 @@ if PCH_GBE
 config PCH_PTP
 	bool "PCH PTP clock support"
 	default n
-	depends on PTP_1588_CLOCK_PCH
+	select PTP_1588_CLOCK_PCH
 	---help---
 	  Say Y here if you want to use Precision Time Protocol (PTP) in the
 	  driver. PTP is a method to precisely synchronize distributed clocks