net: pktgen: replace ENOTSUPP with EOPNOTSUPP

Replace ENOTSUPP with EOPNOTSUPP, fixes checkpatch hint

  WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP

and e.g.

  $ echo "clone_skb 1" > /proc/net/pktgen/lo\@0
  -bash: echo: write error: Unknown error 524

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250219084527.20488-2-ps.report@gmx.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>