parser: use signed char for signal

char defaults to unsigned on some platforms...
fix channel flags A/B/G

Not sure how IEEE80211_CHAN_DYN is used nowadays... Seems like HT/VHT:

from mac80211/rx.c:

	else if (status->flag & (RX_FLAG_HT | RX_FLAG_VHT))
		channel_flags |= IEEE80211_CHAN_DYN | IEEE80211_CHAN_2GHZ;