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>