net: stmmac: dwmac-stm32: update err status in case different of stm32mp13

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>