cxgb4: flower: use NL_SET_ERR_MSG_MOD for validation errors

cxgb4: flower: use NL_SET_ERR_MSG_MOD for validation errors

Replace netdev_{warn,err} with NL_SET_ERR_MSG_{FMT_,}MOD
to better inform the user about the problem.

Only compile-tested, no access to HW.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://lore.kernel.org/r/20240408165506.94483-1-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>