[PATCH] ibmveth: Fix index increment calculation

The recent commit 751ae21c6cd1493e3d0a4935b08fb298b9d89773 introduced a bug
in the producer/consumer index calculation in the ibmveth driver -
incautious use of the post-increment ++ operator resulted in an increment
being immediately reverted.  This patch corrects the logic.

Without this patch, the driver oopses almost immediately after activation
on at least some machines.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Acked-by: Santiago Leon <santil@us.ibm.com>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>