codel: fix kernel-doc notation warnings

Use '@' before the struct member names in kernel-doc notation
to prevent kernel-doc warnings.

codel.h:158: warning: Function parameter or member 'ecn_mark' not described in 'codel_stats'
codel.h:158: warning: Function parameter or member 'ce_mark' not described in 'codel_stats'

Fixes: 76e3cc12 ("codel: Controlled Delay AQM")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Link: https://lore.kernel.org/r/20230714045127.18752-4-rdunlap@infradead.org


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