mwl8k: choose proper firmware image as directed by user

mwl8k: choose proper firmware image as directed by user

The mwl8k can operate in AP or STA mode, depending on the
firmware image that is loaded.  By default, STA firmware is
loaded.  Allow the user to override this default mode at
module load time.  This saves an unnecessary firmware reload
for users only interested in AP mode.

Also, the firmware image can be swapped to meet the user's
add_interface request.  For example, suppose the STA
firmware is loaded, no STA interface has been added, and the
user adds an AP interface.  In this case, the AP firmware
will be loaded to meet the request.

Based on contributions from Pradeep Nemavat <pnemavat@marvell.com>,
Yogesh Powar <yogeshp@marvell.com>, and
Lennert Buytenhek <buytenh@wantstofly.org>.

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