iwlwifi: mvm: fix endianity issues with Smart Fifo commands

This code was broken on big endian systems. Sparse didn't
catch the bug since the firmware command was not tagged as
little endian.
Fix the bug for big endian systems and tag the field in the
firmware command to prevent such issues in the future.

Cc: stable@vger.kernel.org [3.14+]
Fixes: 1f3b0ff8 ("iwlwifi: mvm: Add Smart FIFO support")
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>