net: add missing net_device::xdp_zc_max_segs description

Cited commit under 'Fixes' tag introduced new member to struct
net_device without providing description of it - fix it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20230720141613.61488b9e@canb.auug.org.au/
Fixes: 13ce2daa

 ("xsk: add new netlink attribute dedicated for ZC max frags")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Simon Horman <simon.horman@corigine.com> # build-tested
Link: https://lore.kernel.org/r/20230721145808.596298-1-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>