net: mscc: ocelot: check return values of writes during reset

The ocelot_reset() function utilizes regmap_field_write() but wasn't
checking return values. While this won't cause issues for the current MMIO
regmaps, it could be an issue for externally controlled interfaces.

Add checks for these return values.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>