i2c-au1550: Fix PM support

Fix driver power management:
- suspend the PSC while driver is idle.
- move PSC init/deinit to separate functions, as PSC must be
  initialized/shutdown on resume/suspend.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>