mlxsw: item: Annotate item helpers with '__maybe_unused'

mlxsw is using helpers to get / set fields in messages exchanged with
the device. It is possible that some fields are only set or only get.
This causes LLVM to emit warnings such as the following when building
with W=1 [1]:

drivers/net/ethernet/mellanox/mlxsw/core_acl_flex_actions.c:2022:1: warning: unused function 'mlxsw_afa_sampler_mirror_agent_get'

The fact that some fields are only set or only get is very much
intentional and not indicative of functions that need to be removed.

Therefore, annotate the item helpers with '__maybe_unused' to suppress
these warnings.

[1] https://lkml.org/lkml/2021/9/29/685

Cc: Nathan Chancellor <nathan@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20211008132315.90211-1-idosch@idosch.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>