bwn: fixing performance problems and an RX buffer overflow problem

bwn: fixing performance problems and an RX buffer overflow problem

* Commenting a bwn_shm_write_2 call that destroys performance especially
  when sending.  This call is also commented in Linux's b43 driver.

* Handling Overflows of the RX buffer in bwn by just dropping all entries.
  With 256 slots in the RX buffer this overflow will almost never happen in
  practice.