iwlegacy: make il3945_mac_ops __ro_after_init

There's no need for this to be only __read_mostly, since
it's only written in a single way depending on the module
parameter, so that can be moved into the module's __init
function, and the ops can be __ro_after_init.

This is a little bit safer since it means the ops can't
be overwritten (accidentally or otherwise), which would
otherwise cause an arbitrary function or bad pointer to
be called.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>