net: enetc: refactor enetc_setup_tc_taprio() to have a switch/case for cmd

Make enetc_setup_tc_taprio() more amenable to future extensions, like
reporting statistics.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>