atm: fore200e: Fix build warning.

GCC (rightfully) complains that:

drivers/atm/fore200e.c:614:5: warning: operation on 'cmdq->head' may be undefined

This is due to the FORE200E_NEXT_ENTRY macro, which essentially
evaluates to:

	i = ++i % m

Make it what's explicitly intended here which is:

	i = (i + 1) % m

and the warning goes away.

Signed-off-by: David S. Miller <davem@davemloft.net>