uio: fix wrong return value from uio_mmap()

uio_mmap has multiple fail paths to set return value to nonzero then
goto out. However, it always returns *0* from the *out* at end, and
this will mislead callers who check the return value of this function.

Fixes: 57c5f4df ("uio: fix crash after the device is unregistered")
CC: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Hailong Liu <liu.hailong6@zte.com.cn>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>