net: stmmac: unexport stmmac_init_tstamp_counter()

Nothing outside of stmmac_main.c makes use of
stmmac_init_tstamp_counter(), so there's no point exporting it for
modules, or even having it non-static. Remove the export and make
it static.

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