enetc: Use generic rule to map Tx rings to interrupt vectors

Even if the current mapping is correct for the 1 CPU and 2 CPU cases
(currently enetc is included in SoCs with up to 2 CPUs only), better
use a generic rule for the mapping to cover all possible cases.
The number of CPUs is the same as the number of interrupt vectors:

Per device Tx rings -
device_tx_ring[idx], where idx = 0..n_rings_total-1

Per interrupt vector Tx rings -
int_vector[i].ring[j], where i = 0..n_int_vects-1
			     j = 0..n_rings_per_v-1

Mapping rule -
n_rings_per_v = n_rings_total / n_int_vects
for i = 0..n_int_vects - 1:
	for j = 0..n_rings_per_v - 1:
		idx = n_int_vects * j + i
		int_vector[i].ring[j] <- device_tx_ring[idx]

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210409071613.28912-1-claudiu.manoil@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>