net: stmmac: Make ptp_clk_freq_config variable type explicit

The priv variable is _always_ of type (struct stmmac_priv *), so let's
stop using (void *) since it isn't abstracting anything.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20230725211853.895832-3-ahalaney@redhat.com


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