iwlwifi: mvm: check for short GI only for OFDM

[ Upstream commit 4c59ff5a9a9c54cc26c807dc2fa6933f7e9fa4ef ]

This bit will be used in CCK to indicate short preamble.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>