ath_rx_init() demonstrates some serious confusion over how to use
pointers, resulting in (uint32_t*)NULL being used as a temporary
variable.  This does not end well.

The broken code in question is performing manual alignment of I/O
buffers, which can now be achieved more simply using alloc_iob_raw().
Fix by removing ath_rxbuf_alloc() entirely.

Signed-off-by: Michael Brown <mcb30@ipxe.org>