b43: Fix firmware caching

We must also store the ID string (filename) for the cached firmware blobs
and verify that we really have the right firmware cached before using it.
If we don't have the right fw cached, we must free it and request the
correct blobs.

This fixes bandswitch on A/B/G multi-PHY devices.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>