number of TX memory blocks minus the allocated ones.

Also, remove some unnecessary castings and use union member accesses
instead.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>
FR)