iwlwifi: change byte count table for a000 devices

Since TFD was enlarged to 256 bytes, the fetch of the TFD
itself is very expensive.
To make DRAM to SRAM more efficient, bits 12-13 will indicate
the number of 64 byte chunks that should be transferred to
SRAM.

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