USB: cp210x: remove useless set memory to zero use memset()

The memory return by kzalloc() or kmem_cache_zalloc() has already
be set to zero, so remove useless memset(0).

spatch with a semantic match is used to found this problem.
(http://coccinelle.lip6.fr/)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>