iwlegacy: fix tx_power initialization

priv->tx_power_next is not initialized to max supported power,
but instead default value is used, what cause errors like

[   58.597834] iwl3945 0000:03:00.0: Requested user TXPOWER 15 above upper limit 14.
[   58.597839] iwl3945 0000:03:00.0: Error setting Tx power (-22).

if maximum tx power read from the eeprom is smaller than default.
In consequence card is unable to initialize properly. Fix the problem
and cleanup tx power initialization.

Reported-and-tested-by: Robin Dong <hao.bigrat@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>