driver: hv: remove cast for kmalloc return value

remove cast for kmalloc return value.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: devel@linuxdriverproject.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>