iwl3945: fix "‘power_idx’ may be used uninitialized" warning

drivers/net/wireless/iwlwifi/iwl-3945.c: In function ‘iwl3945_txpower_set_from_eeprom’:
drivers/net/wireless/iwlwifi/iwl-3945.c:2222: warning: ‘power_idx’ may be used uninitialized in this function

Signed-off-by: John W. Linville <linville@tuxdriver.com>