mwifiex: Remove casting the return value which is a void pointer

Casting the return value which is a void pointer is redundant.
The conversion from void pointer to any other pointer type is
guaranteed by the C programming language.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>