bnx2x: organize BDs calculation for stop/resume

Put the numbers used for stop/resume queue in a single place and
fix the condition for sanity check.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>