net: stmmac: dwmac-stm32: Add test to verify if ETHCK is used before checking clk rate

When we want to use clock from RCC to clock Ethernet PHY (with ETHCK)
we need to check if value of clock rate is authorized.
If ETHCK is unused, the ETHCK frequency is 0Hz and validation fails.
It makes no sense to validate unused ETHCK, so skip the validation.

Fixes: 582ac134 ("net: stmmac: dwmac-stm32: Separate out external clock rate validation")
Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>