char: xillybus: Fix internal data structure initialization

A couple of fields in a data structure, which is used by the driver only,
were not initialized properly during the driver's setup.

The primary issue with this bug was that channel->wr_buf_size remained zero,
so calls to dma_sync_single_for_cpu() took place with zero size, and
consequently did nothing.

This had a rather minimal practical impact, because

(a) these calls are NOPs on Intel/AMD platforms, as well as other platforms
    with coherent cache, and
(b) it's extremely rare that any cache line would survive between two reads
    from a given DMA buffer

Hence no significant practical difference is expected with this patch.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>