[ARM] uengine: fix memset size error

The size passing to memset is wrong.  And here we can replace kmalloc with
kzalloc.

Signed-off-by Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>