selftests: drv-net: gro: use cmd print

selftests: drv-net: gro: use cmd print

Now that cmd() can be printed directly remove the old formatting.

Before:

  # fragmented ip6 doesn't coalesce:
  # Expected {200 100 100 }, Total 3 packets
  # Received {200 100 }, Total 2 packets.
  # /root/ksft-net-drv/drivers/net/gro: incorrect number of packets

Now:

  # CMD: drivers/net/gro --ipv6 --dmac 9e:[...]
  #   EXIT: 1
  #   STDOUT: fragmented ip6 doesn't coalesce:
  #   STDERR: Expected {200 100 100 }, Total 3 packets
  #           Received {200 100 }, Total 2 packets.
  #           /root/ksft-net-drv/drivers/net/gro: incorrect number of packets

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260113000740.255360-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>