iwlwifi: mvm: update values for Smart Fifo

Interoperability issues were identified and root caused to
the Smart Fifo watermarks. These issues arose with
NetGear R7000. Fix this.

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>