qed: Pass src/dst sizes when interacting with MFW

The driver interaction with management firmware involves a union
of all the data-members relating to the commands the driver prepares.

Current interface assumes the caller always passes such a union -
but thats cumbersome as well as risky [chancing a stack corruption
in case caller accidentally passes a smaller member instead of union].

Change implementation so that caller could pass a pointer to any
of the members instead of the union.

Signed-off-by: Tomer Tayar <Tomer.Tayar@cavium.com>
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>