net: stmmac: ptp: conditionally populate getcrosststamp() method

drivers/char/ptp_chardev.c::ptp_clock_getcaps() uses the presence of
the getcrosststamp() method to indicate to userspace whether
crosststamping is supported or not. Therefore, we should not provide
this method unless it is functional. Only set this method pointer
in stmmac_ptp_register() if the platform glue provides the
necessary functionality.

This does not mean that it will be supported (see intel_crosststamp(),
which is the only implementation that may have support) but at least
we won't be suggesting that it is supported on many platforms where
there is no hope.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uto2d-00000001se4-0JSY@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>