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.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>