Fixes a warning that occurs when rc table support is enabled
(IEEE80211_HW_SUPPORTS_RC_TABLE) in mac80211_hwsim and the PS mode
is changed via the exported debugfs attribute.

When the PS mode is changed, a packet is broadcasted via
hwsim_send_nullfunc by creating and transmitting a plain skb with only
header initialized. The ieee80211 rate array in the control buffer is
zero-initialized. When ratetbl support is enabled, ieee80211_get_tx_rates
is called for the skb with sta parameter set to NULL and thus no
ratetbl can be used. The final rate array then looks like
[-1,0; 0,0; 0,0; 0,0] which causes the warning in ieee80211_get_tx_rate.

The issue is fixed by setting the count of the first rate with idx '0'
to 1 and hence ieee80211_get_tx_rates won't overwrite it with idx '-1'.
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>