iwlwifi: support "pure 40MHz" in RXON command

Fix the bug when using 11n "pure 40MHz" mode cause uCode
crashing by adding support for "pure 40MHz" in RX_ON command flag.
the "mode" field (bits 25:26) has value of 0-3
    0 = 20 MHz only
    1 = 40MHz only
    2 = Mixed
    3 = Reserved
Control Channel ID (bit 22) is valid only in Mixed mode.

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