ntb: Check tx descriptors outstanding instead of head/tail for tx queue

Use existing function ntb_transport_tx_free_entry() instead of open coding
the check to see if there are outstanding tx descriptors.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>