mac80211_hwsim: Fix radio dump for radio idx 0

Since 6335698e the radio with idx of 0
will not get dumped in HWSIM_CMD_GET_RADIO because of the last_idx
checks.  Offset cb->args[0] by 1 similarly to what is done in nl80211.c.

Fixes: 6335698e ("mac80211_hwsim: add generation count for netlink dump operation")
Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>