pinsched: Report queued packet count correctly.

'n_txq' was initialized to 0 and never modified, so pinsched_count_txqlen()
always returned 0.  Instead, return the correct number.

This only affected the results of "ovs-appctl memory/show", and only if
controller rate limiting was turned on, so it is not a serious bug.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>