iwlwifi: increase max tfd payload size

Increase the size of TFD_MAX_PAYLOAD_SIZE (the size of iwl_device_cmd)
to accommodate iwl6000_channel_switch_cmd data structure.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Acked-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>