selftests/xsk: Ensure traffic validation proceeds after ring size adjustment in xskxceiver

Previously, HW_SW_MIN_RING_SIZE and HW_SW_MAX_RING_SIZE test cases were
not validating Tx/Rx traffic at all due to early return after changing HW
ring size in testapp_validate_traffic().

Fix the flow by checking return value of set_ring_size() and act upon it
rather than terminating the test case there.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20240702055916.48071-2-tushar.vyavahare@intel.com