The connector driver never modifies any cb_id passed to it, so add a const
qualifier to those arguments so callers can declare their struct cb_id as a
constant object.

Fixes build warnings like these when passing a constant struct cb_id:

  warning: passing argument 1 of ‘cn_add_callback’ discards ‘const’ qualifier from pointer target
Signed-off-by: Geoff Levand <geoff@infradead.org>
Link: https://lore.kernel.org/r/a9e49c9e-67fa-16e7-0a6b-72f6bd30c58a@infradead.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>