bnx2x: Fix UNDI driver unload

Commit 91ebb928b "bnx2x: Add support for Multi-Function UNDI" contains a bug
which prevent the emptying of the device's Rx buffers before reset.
As a result, on new boards it is likely HW will reach some fatal assertion
once its interfaces load after UNDI was previously loaded.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>