mwifiex: remove casts of void pointers

In some cases local pointers are used to cast void pointers passed to
the function. Those unnecessary local pointers are also removed.

This patch was inspired by Joe Perches' patch
[PATCH net-next 1/2] wireless: Remove casts of void *;
and the comments from Julian Calaby.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Cc: Joe Perches <joe@perches.com>
Cc: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>