iwinfo: print HT operation only if present

iwinfo: print HT operation only if present

HT Operation is empty in the 6GHz band, where only 802.11 ax (HE)
and 802.11 be (EHT) are allowed, and in 802.11 legacy modes.

Example:
Cell 19 - Address: xx:xx:xx:xx:xx:xx
          ESSID: "OpenWrt"
          Mode: Master  Frequency: 5.955 GHz  Band: 6 GHz  Channel: 1
          Signal: -20 dBm  Quality: 70/70
          Encryption: WPA3 SAE (CCMP)
          HT Operation:
                    Primary Channel: 0
                    Secondary Channel Offset: no secondary
                    Channel Width: 20 MHz
          HE Operation:
                    Center Frequency 1: 7
                    Center Frequency 2: 15
                    Channel Width: 160 MHz
          EHT Operation:
                    Center Frequency 1: 15
                    Center Frequency 2: 31
                    Channel Width: 320 MHz

Fixes: ea28dfb5f957 ("iwinfo: export ht and vht operation in scan results")
Signed-off-by: Aleksander Jan Bajkowski <[email protected]>
Link: https://github.com/openwrt/iwinfo/pull/21
Signed-off-by: Robert Marko <[email protected]>