bcma: clean exports of functions

Function managing IRQs is needed for external drivers like b43.
On the other side we do not expect writing any hosts drivers outside of
bcma, so this is safe to do not export functions related to this.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>