ath5k: Preserve tx power level requested from above on phy_init

By using cur_pwr on phy_init we re-use the power level previously set by the
driver, not the one we got from above.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>