ssb: add 'ssb_pcihost_set_power_state' function

This patch adds the 'ssb_pcihost_set_power_state' function.

This function allows us to set the power state of a PCI device
(for example b44 ethernet device).

Signed-off-by: Miguel Botón <mboton@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>