mac80211: fix use of local variable

mac80211_get_addr is called from mac80211_generate_mac, where the local variable
initialisation id="${macidx:-0}" suggests that macidx is not always defined.
Probably, idx was supposed to be used instead of $(($macidx + 1)).

Fixes: 4d99db168cf7 ("mac80211: try to get interface addresses from wiphy sysfs 'addresses' if no mask is set")
Signed-off-by: Leon M. George <leon@georgemail.eu>