nl80211: Change max TX power to be in mBm instead of dBm

In order to be consistent with NL80211_ATTR_POWER_RULE_MAX_EIRP,
change NL80211_FREQUENCY_ATTR_MAX_TX_POWER to use mBm and U32 instead
of dBm and U8. This is a userspace interface change, but the previous
version had not yet been pushed upstream and there are no userspace
programs using this yet, so there is justification to get this change in
as long as it goes in before the previous version gets out.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>