mwl8k: handle loading AP firmware images

AP and STA firmware images provide a different signature in the
HIU_INT_CODE register after loading.  Record which of the signatures
we saw, as it determines which command sequences to use later on.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>