iwinfo: nl80211: fix hwmode parsing for multi-band NICs

iwinfo: nl80211: fix hwmode parsing for multi-band NICs

In case a NIC supports multiple frequency bands, the supported
hw-modelist might not contain all valid hwmodes, as B/G/AD hwmodes are
only included based on the last parsed supported frequency.

In case a radio supports multiple bands, this might result in these
hwmodes not being flagged as supported.

Circumvent this by tracking all seen frequency bands using a bitmask
which later determined which HW modes are listed as supported.

Signed-off-by: David Bauer <mail@david-bauer.net>