If the driver indicates capability for a band that
hostapd/wpa_supplicant does not support, the struct hostapd_hw_modes
array of bands got an empty entry for that with NUM_HOSTAPD_MODES as the
mode. This resulted in various issues, e.g., with fst_hw_mode_to_band()
hitting a WPA_ASSERT(0).
Fix this by filtering out unsupported bands from the internal data
structures.
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>