iwlwifi: Fix typo in iwl-commands.h for CCK rate bit range.

My first (minor) patch, hopefully this is correct.

Fix a typo in iwl-commands.h for CCK rates which needs 7 bits and not 4.

Signed-off-by: Daniel Wu <dyqith@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>