pch_gbe, ptp_pch: Fix the dependency direction between these drivers

In commit a24006ed12616bde1bbdb26868495906a212d8dc ('ptp: Enable clock
drivers along with associated net/PHY drivers') I wrongly made
PTP_1588_CLOCK_PCH depend on PCH_GBE.  The dependency is really the
other way around.  Therefore make PCH_GBE select PTP_1588_CLOCK_PCH
and remove the 'default y' from the latter.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>