net: ethernet: ti: am65-cpsw: Fix NULL dereference on XDP_TX

If number of TX queues are set to 1 we get a NULL pointer
dereference during XDP_TX.

~# ethtool -L eth0 tx 1
~# ./xdp-trafficgen udp -A <ipv6-src> -a <ipv6-dst> eth0 -t 2
Transmitting on eth0 (ifindex 2)
[  241.135257] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030

Fix this by using actual TX queues instead of max TX queues
when picking the TX channel in am65_cpsw_ndo_xdp_xmit().

Fixes: 8acacc40 ("net: ethernet: ti: am65-cpsw: Add minimal XDP support")
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Julien Panis <jpanis@baylibre.com>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>