bnx2x: Avoid sending multiple statistics queries

During traffic when DCB is enabled, it is possible for multiple instances
of statistics queries to be sent to the chip - this may cause the FW to assert.

This patch prevents the sending of an additional instance of statistics query
while the previous query hasn't completed.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>