tg3: Fix return ring size breakage

tg3: Fix return ring size breakage

Commit 83226863377b446d067b65a640c00fd747458c42, "tg3: Add 5717 asic
rev" changed how the rx return ring size operations are done.  It
effectively inverts the sense of the previous test, but it failed to
also invert the resulting sizes.  This patch corrects that error.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>