rtc-pcf8563-guf: prevent resetting of TIE bit to support bootselect

Save the value of TIE bit value before resetting CONTROL_STATUS_2 register in
probe() routine of PCF8563 driver. Otherwise, a bootselect functionality stops
working because it stores some information across reboot in RTC registers.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>