avr32: Fix typo in read_persistent_clock()

Fix fallout from the persistent_clock changes.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>