iwlwifi: fix incorrect 5GHz rates reported in monitor mode

This patch fixes the rates reported in monitor mode operation
(Wireshark) for iwlwifi.

Previously, packets with rates of 6M..24M would be reported
incorrectly and packets with rates of 36M..54M would not passed
up the stack.

Signed-off-by: Rick Farrington <rickdic@hotmail.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>