mlxsw: Make mailboxes 4KB aligned

The HW-SW contract requires mailboxes passed to the firmware to be 4KB
aligned. Previously, these mailboxes were mapped using streaming DMA
routines, which do not guarantee the bus addresses to be 4KB aligned.
Under certain conditions this constraint was indeed violated and errors
were observed.

By using consistent DMA mapping routines together with a mailbox size of
4KB we are guaranteed not to violate the constraint.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>