mac80211: Fix setting txpower to zero

With multiple VIFS ath10k, and probably others, tries to find the
minimum txpower for all vifs and uses that when setting txpower in
the firmware.

If a second vif is added and starts to scan, it's txpower is not
initialized yet and it set to zero.

ath10k had a patch to ignore zero values, but then it is impossible
to actually set txpower to zero.

So, instead initialize the txpower to INT_MIN in mac80211, and let
drivers know that means the power has not been set and so should
be ignored.

This should fix regression in:

commit 88407beb


Author: Ryan Hsu <ryanhsu@qca.qualcomm.com>
Date:   Tue Dec 13 14:55:19 2016 -0800

    ath10k: fix incorrect txpower set by P2P_DEVICE interface

Tested on ath10k 9984 with ath10k-ct firmware.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Link: https://lore.kernel.org/r/20191217183057.24586-1-greearb@candelatech.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>