bnx2x: Control number of vfs dynamically

1. Support sysfs interface for getting the maximal number of virtual functions
   of a given physical function.
2. Support sysfs interface for getting and setting the current number of
   virtual functions.

Signed-off-by: Ariel Elior <ariele@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>