b43: Refresh RX poison on buffer recycling

The RX buffer poison needs to be refreshed, if we recycle an RX buffer,
because it might be (partially) overwritten by some DMA operations.

Cc: stable@kernel.org
Cc: Francesco Gringoli <francesco.gringoli@ing.unibs.it>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>