ath5k: more RF2413 stuff

 * Add AR5K_RF2413 to radio check during hw_reset so it doesn't complain

 * Write ah_phy_spending value we set during attach instead of checking
each time for radio revision

 * Skip txpower setup for RF2413 because it can't transmit with it
(weird thing is that RF5413 has no problem with it).

Changes-licensed-under: ISC
Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>