platform/surface: aggregator: Fix access of unaligned value

The raw message frame length is unaligned and explicitly marked as
little endian. It should not be accessed without the appropriate
accessor functions. Fix this.

Note that payload.len already contains the correct length after parsing
via sshp_parse_frame(), so we can simply use that instead.
Reported-by: kernel-test-robot <lkp@intel.com>
Fixes: c167b9c7 ("platform/surface: Add Surface Aggregator subsystem")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Acked-by: Mark Gross <mgross@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210211124149.2439007-1-luzmaximilian@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/surface/aggregator/ssh_packet_layer.c

@@ -1774,7 +1774,7 @@ static size_t ssh_ptl_rx_eval(struct ssh_ptl *ptl, struct ssam_span *source)
 		break;
 	}
 
-	return aligned.ptr - source->ptr + SSH_MESSAGE_LENGTH(frame->len);
+	return aligned.ptr - source->ptr + SSH_MESSAGE_LENGTH(payload.len);
 }
 
 static int ssh_ptl_rx_threadfn(void *data)