net: stmmac: fix typo in comment

This is just a trivial fix for a typo in a comment, no functional
changes.

Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20240228112447.1490926-1-siyanteng@loongson.cn
Signed-off-by: Paolo Abeni <pabeni@redhat.com>