ath5k: Simplify powertable recalculation

Let ath5k_hw_txpower() decide if it can re-use the powertable or if it has to
be recalculated instead of passing a 'fast' flag from the outside.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>