ath9k: correct expected max RX buffer size

We should only tell the hardware its capable of DMA'ing
to us only what we asked dev_alloc_skb(). Prior to this
it is possible a large RX'd frame could have corrupted
DMA data but for us but we were saved only because we
were previously also pci_map_single()'ing the same large
value. The issue prior to this though was we were unmapping
a smaller amount which the prior DMA patch fixed.

Signed-off-by: Bennyam Malavazi <Bennyam.Malavazi@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>