iwlwifi: mvm: read IWL_RX_MPDU_PHY_SHORT_PREAMBLE only for CCK

Due to a general shortage of RX API bits, the firmware is going
to reuse this bit on non-CCK frames to mean something else. Use
it only on CCK frames to prepare for that change.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>