iwlwifi: pcie: properly configure the debug buffer size for 8000

8000 device family has a new debug engine that needs to be
configured differently than 7000's.
The debug engine's DMA works in chunks of memory and the
size of the buffer really means the start of the last
chunk. Since one chunk is 256-byte long, we should
configure the device to write to buffer_size - 256.
This fixes a situation were the device would write to
memory it is not allowed to access.

CC: <stable@vger.kernel.org> [4.1+]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>