net: dsa: mv88e6xxx: depend on PTP conditionally

PTP hardware timestamping related objects are not linked when PTP
support for MV88E6xxx (NET_DSA_MV88E6XXX_PTP) is disabled, therefore
NET_DSA_MV88E6XXX should not depend on PTP_1588_CLOCK_OPTIONAL
regardless of NET_DSA_MV88E6XXX_PTP.

Instead, condition more strictly on how NET_DSA_MV88E6XXX_PTP's
dependencies are met, making sure that it cannot be enabled when
NET_DSA_MV88E6XXX=y and PTP_1588_CLOCK=m.

In other words, this commit allows NET_DSA_MV88E6XXX to be built-in
while PTP_1588_CLOCK is a module, as long as NET_DSA_MV88E6XXX_PTP is
prevented from being enabled.

Fixes: e5f31552 ("ethernet: fix PTP_1588_CLOCK dependencies")
Signed-off-by: Johnny S. Lee <foss@jsl.io>
Signed-off-by: David S. Miller <davem@davemloft.net>