net: stmmac: rename stmmac_init_ptp()

Changes to the stmmac driver to fix various issues with PTP have made
stmmac_init_ptp() less about initialising the entire PTP block, and
now primarily deals with the packet timestamping support. The exception
to this is ptp_clk_freq_config(), which is an odditiy. It remains
as stmmac_init_ptp() is used both at .ndo_open() time and in the
resume paths.

However, restructuring this code to make it more easily readable makes
the continued use of "init_ptp" confusing.

In preparation to cleaning up the (re-)initialisation of timestamping,
rename the existing stmmac_init_ptp() to stmmac_init_timestamping()
which better reflects its functionality.

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