Staging: wlan-ng: Update prism2_set_tx_power() to use mBm

Commit fa61cf70 updates nl80211's function
.set_tx_power to use mBm units instead of dBm for greater accuracy and
smaller power levels.

This patchs updates prism2_set_tx_power() accordingly.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>