carl9170 firmware: fix max beacon length

carl9170 firmware: fix max beacon length

According to AR9170's data sheet, the
beacon length register only cares about
bits 8:0. So the max length [with 4
reserved bytes for the FCS] is at 511.

Note:

Because the beacon has to be stored in the
DMA memory region, we have to be careful
so we don't waste precious memory due to
alignment and padding. Therefore I decided
to reduce the max beacon length even more
down to 480 bytes. [This saves one 320 byte
DMA block from becoming unused padding, when
the default 2 vif configuration is selected].

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>