rt2x00: read 5GHz TX power values from the correct offset

The current code uses the same index value both
for the channel information array and for the TX
power table. The index starts from 14, however the
index of the TX power table must start from zero.

Fix it, in order to get the correct TX power value
for a given channel.

The changes in rt61pci.c and rt73usb.c are compile
tested only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: stable@vger.kernel.org
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>