staging: et131x: zero allocation of fbr to prevent random address access

If et131x_rx_dma_memory_alloc() allocates rx_ring->fbr[0] but fails to
allocate rx_ring->fbr[1], this leaves fbr[0]->ring_virtaddr with the
possibility of being accessed in et131x_rx_dma_memory_free() as it
contains a random value, potentially causing an oops.

Fix this by zeroing the fbr memory on allocation. Subsequent frees of
this fbr memory explicitly zeros the ring_virtaddr value.

Reported-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>