mwifiex: replace unnecessary u32 variables with u8 in sdio.c

Some u32 variables in sdio.c are used to store/pass u8 values.
Replace them with u8 variables.

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>