net: mscc: ocelot: utilize readx_poll_timeout() for chip reset

Clean up the reset code by utilizing readx_poll_timeout instead of a custom
loop.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>