wl12xx: Simplify TX blocks accounting

The total number of TX memory blocks may change when the dynamic memory
option is enabled. The current implementation only tracks the available
memory blocks, which over-complicates TX blocks accounting.

By tracking the number of allocated blocks, calculation of the number of
available blocks becomes simpler and cleaner. It simply equals the total
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>