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>

drivers/net/dsa/mv88e6xxx/Kconfig

@@ -2,7 +2,6 @@
 config NET_DSA_MV88E6XXX
 	tristate "Marvell 88E6xxx Ethernet switch fabric support"
 	depends on NET_DSA
-	depends on PTP_1588_CLOCK_OPTIONAL
 	select IRQ_DOMAIN
 	select NET_DSA_TAG_EDSA
 	select NET_DSA_TAG_DSA
@@ -13,7 +12,8 @@ config NET_DSA_MV88E6XXX
 config NET_DSA_MV88E6XXX_PTP
 	bool "PTP support for Marvell 88E6xxx"
 	default n
-	depends on NET_DSA_MV88E6XXX && PTP_1588_CLOCK
+	depends on (NET_DSA_MV88E6XXX = y && PTP_1588_CLOCK = y) || \
+	           (NET_DSA_MV88E6XXX = m && PTP_1588_CLOCK)
 	help
 	  Say Y to enable PTP hardware timestamping on Marvell 88E6xxx switch
 	  chips that support it.