mac80211: default to automatic power control

In "mac80211: correct wext transmit power handler"
I fixed the wext handler, but forgot to make the default of the
user_power_level -1 (aka "auto"), so that now the transmit power
is always set to 0, causing associations to time out and similar
problems since we're transmitting with very little power. Correct
this by correcting the default user_power_level to -1.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Bisected-by: Niel Lambrechts <niel.lambrechts@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>