net: pktgen: fix code style (ERROR: space prohibited after that '&')

net: pktgen: fix code style (ERROR: space prohibited after that '&')

Fix checkpatch code style errors/checks:

  CHECK: No space is necessary after a cast
  #2984: FILE: net/core/pktgen.c:2984:
  +       *(__be16 *) & eth[12] = protocol;

  ERROR: space prohibited after that '&' (ctx:WxW)
  #2984: FILE: net/core/pktgen.c:2984:
  +       *(__be16 *) & eth[12] = protocol;

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>