net: stmmac: move PTP support check into stmmac_init_timestamping()

Move the PTP support check from stmmac_init_tstamp_counter() into
stmmac_init_timestamping() as it makes more sense to be there.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>