netifd: improve packet steering on ipq40xx (and possibly others)

netifd: improve packet steering on ipq40xx (and possibly others)

Some platforms a single ethernet device for all ports with multiple rx rings
and NAPI threading enabled. In this case, the steering script was limiting
performance by keeping all NAPI threads assigned to the same CPU.
Fix this by assigning each rx queue and the corresponding NAPI task separately.
Additionally, if the number of rx queues is at least as big as the number of
CPUs, skip weight based assignment and distribute the load across all CPUs
directly.

Fixes: https://github.com/openwrt/openwrt/issues/17611
Signed-off-by: Felix Fietkau <nbd@nbd.name>