iwlwifi: pcie: limit fw chunk sizes given to fh

New FW has chunks that are larger than the size limit of the
FH's DMA. To make sure we don't crash it - actively limit the
max size of each chunk.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>