ssi: change ssi_slave_init to be a realize ops

ssi: change ssi_slave_init to be a realize ops

This enables qemu to handle late inits and report errors. All the SSI
slave routine names were changed accordingly. Code was modified to
handle errors when possible (m25p80 and ssi-sd)

Tested with the m25p80 slave object.

Suggested-by: Paolo Bonzini <[email protected]>
Signed-off-by: Cédric Le Goater <[email protected]>
Message-id: 1467138270[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>