hw/arm/sysbus-fdt: Fix assertion in copy_properties_from_host()

When copy_properties_from_host() ignores the error for an optional
property, it frees the error, but fails to reset it.

Hence if two or more optional properties are missing, an assertion is
triggered:

    util/error.c:57: error_setv: Assertion `*errp == NULL' failed.

Fis this by resetting err to NULL after ignoring the error.

Fixes: 9481cf2e ("hw/arm/sysbus-fdt: helpers for clock node generation")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Message-id: 20180725113000.11014-1-geert+renesas@glider.be
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>