iwlwifi: pcie: generalize and increase the size of scratchbuf

Currently the scratch buffer is set to 16 bytes and indicates
the size of the bi-directional DMA.
However, next HW generation will perform additional offloading,
and will write the result in the key location of the TX command,
so the size of the bi-directional consistent memory should grow
accordingly - increase it to 40.
Generalize the code to get rid of now irrelevant scratch references.

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