b43: improve TX power handling for LP PHYs

I implemented the functions:
"lpphy_update_tx_power_npt"
and
"lpphy_set_tx_power_control",

and together, both improve the bitrate of my device:

wlan0 IEEE 802.11bg ESSID:"tommy"
Mode:Managed Frequency:2.412 GHz Access Point: XXX
Bit Rate=54 Mb/s Tx-Power=20 dBm
Retry long limit:7 RTS thr:off Fragment thr:off
Encryption key:off
Power Management:off
Link Quality=70/70 Signal level=10 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0

without these patches, I maxed out at about 24 Mbit/s.