mwifiex: use u32 variables for SDIO read/write port bitmap

Currently supported SDIO chipsets (SD87XX) have 16 ports.
This change is a prerequisite for new chipset.

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