ath5k: Add RF2413 srev values

 * Add RF2413 srev values and a new entry on ath5k_radio enum for it
  since it differs from RF5413 (it's not like 5112-2112).

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>