Ath5k: fix dma operation

Don't sync
- coherent mapping (descriptors)
- before unmap, it's useless
- (wrongly anyway -- for_cpu) beacon skb, it's just mapped,
  so by the device yet

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>