rt2x00: Fix transmit power troubles on some Ralink RT30xx cards

Some cards on Ralink RT30xx chipset not have correctly TX_MIXER_GAIN
value in them EEPROM/EFUSE. In this case, we must use default value,
but always used EEPROM/EFUSE value. As result we have tranmitt power
range from -10dBm to +6dBm instead 0dBm to +16dBm.

Correctly value in EEPROM/EFUSE is one or more for RT3070 and two or
more for other RT30xx chips.

Tested on Canyon CNP-WF518N1 usb Wi-Fi dongle and Jorjin WN8020 usb
embedded Wi-Fi module.

Signed-off-by: Alex A. Mihaylov <minimumlaw@rambler.ru>
Cc: stable@vger.kernel.org
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>