mwl8k: use kcalloc instead of kmalloc & memset

Use kcalloc or kzalloc rather than the combination of kmalloc and memset.

Thanks coccicheck for detecting this.
(http://coccinelle.lip6.fr/

)

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>