iwlwifi: pcie: prepare for dynamic queue allocation

In a000 transport we will allocate queues dynamically.
Right now queue are allocated as one big chunk of memory
and accessed as such.
The dynamic allocation of the queues will require accessing
the queues as pointers.
In order to keep simplicity of pre-a000 tx queues handling,
keep allocating and freeing the memory in the same style,
but move to access the queues in the various functions as
individual pointers.
Dynamic allocation for the a000 devices will be in a separate
patch.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>