rt2x00: rt2800lib: fix default TX power values for RT3593

The TX power values in the EEPROM are using
a different format for the RT3593 chip. The
default TX power value uses bits 0..4 only.
Bits 5..8 contains value for fine grained
power control. Additionally, the lower and
upper limits of the TX power values are the
same for both bands.

Improve the rt2800_txpower_to_dev function,
in order to compute the correct default power
values for the RT3593 chip as well.

Signed-off-by: Gabor Juhos <juhosg@openwrt.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>