rtc: pcf85063: Always clear EXT_TEST from set_time

Power-on reset after the insertion of a battery does not always complete
successfully, leading to corrupted register content. The EXT_TEST bit
will stop the clock from running, but currently the driver will never
recover.

Safely handle the erroneous state by clearing EXT_TEST as part of the
usual set_time method.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>