wct4xxp: t4_serial_setup() was called more often than necessary.

The driver iterates through all the spans on a given device during assignment,
checking for unassigned spans, but it was erroneously testing the span on which
assigned was called.

This just removes some unexpected behavior and provides a slight performance
increase on load and does not impact the functionality of the driver as far as
I'm aware.

Reported-by: Doug Bailey <dbailey@digium.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>

drivers/dahdi/wct4xxp/base.c

@@ -2059,7 +2059,7 @@ static void t4_span_assigned(struct dahdi_span *span)
 	/* We use this to make sure all the spans are assigned before
 	 * running the serial setup. */
 	list_for_each_entry(pos, &wc->ddev->spans, device_node) {
-		if (!test_bit(DAHDI_FLAGBIT_REGISTERED, &span->flags))
+		if (!test_bit(DAHDI_FLAGBIT_REGISTERED, &pos->flags))
 			++unassigned_spans;
 	}