rtnetlink: remove redundant checks for nlattr IFLA_BRIDGE_MODE

The commit d73ef2d6 ("rtnetlink: let rtnl_bridge_setlink checks
IFLA_BRIDGE_MODE length") added the nla_len check in rtnl_bridge_setlink,
which is the only caller for ndo_bridge_setlink handlers defined in
low-level driver codes. Hence, this patch cleanups the redundant checks in
each ndo_bridge_setlink handler function.

Suggested-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Acked-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230807091347.3804523-1-linma@zju.edu.cn


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