net: micrel: Fix KS8851 Kconfig

KS8851 selects MICREL_PHY, which depends on PTP_1588_CLOCK_OPTIONAL, so
make KS8851 also depend on PTP_1588_CLOCK_OPTIONAL.

Fixes kconfig warning and build errors:

WARNING: unmet direct dependencies detected for MICREL_PHY
Depends on [m]: NETDEVICES [=y] && PHYLIB [=y] && PTP_1588_CLOCK_OPTIONAL [=m]
Selected by [y]:
- KS8851 [=y] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_MICREL [=y] && SPI [=y]

ld.lld: error: undefined symbol: ptp_clock_register referenced by micrel.c
net/phy/micrel.o:(lan8814_probe) in archive drivers/built-in.a
ld.lld: error: undefined symbol: ptp_clock_index referenced by micrel.c
net/phy/micrel.o:(lan8814_ts_info) in archive drivers/built-in.a

Reported-by: kernel test robot <[e-mail]>
Fixes: ece19502834d ("net: phy: micrel: 1588 support for LAN8814 phy")
Signed-off-by: Horatiu Vultur <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/micrel/Kconfig

···
 config KS8851
 	tristate "Micrel KS8851 SPI"
 	depends on SPI
+	depends on PTP_1588_CLOCK_OPTIONAL
 	select MII
 	select CRC32
 	select EEPROM_93CX6