brcm63xx: mac80211.sh handle short preamble
Openwrt option short_preamble is stored per interface and is of type boolean. True means short preamble is allowed, false means long preamble should be used.
Broadcom command plcphdr can get one of those values: auto, long, debug. So maping from short_preamble to plcphdr is as follows: short_preamble=true - plcphdr auto, short_preamble=false - plcphdr long.
Signed-off-by: Marek Puzyniak marek.puzyniak@iopsys.eu