iwlwifi: mvm: Invert the condition for OFDM rate

OFDM rate used for all bands except to band 2.4 which use CCK rate.
Inverting the condition help that in future we won't need to expand the
condition for more bands.

Signed-off-by: Tova Mussai <tova.mussai@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>