iwlwifi: don't export symbols not needed in other modules

Even with the split into iwlcore/agn/3945 not all symbols
that cross file boundaries are needed in other modules, a
few are only used within iwlcore, for example.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>