iwlwifi: increase max payload of iwl_cmd

This patch increases iwl_cmd payload maximum size from 360 to 640
to fit some of iwl5000 commands as well.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>