When CONFIG_BAND_6GHZ and CFG80211_6GHZ_BAND_SUPPORTED are defined in
kernel 4.4, which means 6g feature is enabled, currently there is a
build error: wlan_hdd_iftype_data_alloc_6ghz is defined but not used.
To fix this issue, refine iftype_data_*g defination with below rules.
1. If CONFIG_BAND_6GHZ and CFG80211_6GHZ_BAND_SUPPORTED are defined,
iftype_data_2g, iftype_data_5g and iftype_data_6g will be defined no
matter the kernel version.
2. If the kernel version is not smaller than 4.19, iftype_data_2g and
iftype_data_5g will be defined.
3. If the kernel version is not smaller than 5.4 and CONFIG_BAND_6GHZ
is defined, iftype_data_6g will be defined.
Change-Id: If75bea9fc4c11d6cacd8dbf49455277f055ed8de
CRs-Fixed: 2600962
3d9cd95e51