When the request length is aligned to maxpacketsize, sometimes
the return length ret > the user space requested len.

At that time, we will use min_t(size_t, ret, len) to limit the
size in case of user data buffer overflow.

But we need return the min_t(size_t, ret, len) to tell the user
space rightly also.

[ balbi@ti.com: also fix comment's indentation ]
Acked-by: NMichal Nazarewicz <mina86@mina86.com>
Reviewed-by: NDavid Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: NChuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>