b43: Increase number of RX DMA slots

Bastian Bittorf reported that some of the silent freezes on a Linksys WRT54G
were due to overflow of the RX DMA ring buffer, which was created with 64
slots. That finding reminded me that I was seeing similar crashed on a netbook,
which also has a relatively slow processor. After increasing the number of
slots to 128, runs on the netbook that previously failed now worked; however,
I found that 109 slots had been used in one test. For that reason, the number
of slots is being increased to 256.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Bastian Bittorf <bittorf@bluebottle.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>