net: dsa: microchip: ptp: fix up PTP dependency

When NET_DSA_MICROCHIP_KSZ_COMMON is built-in but PTP is a loadable
module, the ksz_ptp support still causes a link failure:

ld.lld-16: error: undefined symbol: ptp_clock_index
>>> referenced by ksz_ptp.c
>>>               drivers/net/dsa/microchip/ksz_ptp.o:(ksz_get_ts_info) in archive vmlinux.a

This can happen if NET_DSA_MICROCHIP_KSZ8863_SMI is enabled, or
even if none of the KSZ9477_I2C/KSZ_SPI/KSZ8863_SMI ones are active
but only the common module is.

The most straightforward way to address this is to move the
dependency to NET_DSA_MICROCHIP_KSZ_PTP itself, which can now
only be enabled if both PTP_1588_CLOCK support is reachable
from NET_DSA_MICROCHIP_KSZ_COMMON. Alternatively, one could make
NET_DSA_MICROCHIP_KSZ_COMMON a hidden Kconfig symbol and extend the
PTP_1588_CLOCK_OPTIONAL dependency to NET_DSA_MICROCHIP_KSZ8863_SMI as
well, but that is a little more fragile.

Fixes: eac1ea20

 ("net: dsa: microchip: ptp: add the posix clock support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230130131808.1084796-1-arnd@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>