iwlagn: fix tx power initialization

Since

commit f844a709
Author: Stanislaw Gruszka <sgruszka@redhat.com>
Date:   Fri Jan 28 16:47:44 2011 +0100

    iwlwifi: do not set tx power when channel is changing

we set device tx power during initialization to priv->tx_power_next,
which itself is initialized to minimum power. That changed
default behaviour of driver. Previously we initialized device to
transmit at maximum available power by default. Patch change again
to previous behaviour and cleanup tx power initialization.

Fortunately this is not critical fix, as mac80211 layer setup
tx power lately to 14dB, hence device does not operate at minimal
transmit power all the time.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>