mwifiex: define a macro for MPA base address

As Multiple-Port Aggregation base address value is fixed, we can
have a macro for it.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>