The mask parameter of syscfg property is mandatory for MP13 but
optional for all other cases.
The function should not return error code because for non-MP13
the missing syscfg phandle in DT is not considered an error.
So reset err to 0 in that case to support existing DTs without
syscfg phandle.

Fixes: 50bbc039 ("net: stmmac: dwmac-stm32: add management of stm32mp13 for stm32")
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>