bnx2x: Allowing 0 as initial fairness value

Value of zero was used to disable the fairness mechanism. Though the code
(driver and FW) allowed changing the value at run time, it did not allow to do
that if the mechanism was disabled to begin with.
Fixed the FW to allow turning on and off the mechanism at run time. Fixed the
code to read the value from the chip at the right sequence.
Without this fix, if the initial value was set to zero, traffic could not run on
the interface.

Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>