mwl8k: factor out firmware loading and hw init code

This is in preparation for supporting different fw images for
different interface types, and for supporting asynchronous
firmware loading.

Based on a patch from Pradeep Nemavat <pnemavat@marvell.com>
and Yogesh Powar <yogeshp@marvell.com>

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>