rtc: ds1302: fix error check in set_time

The set_time callback for rtc-ds1302 doesn't write clock registers
because the error check for the return value from spi_write_then_read()
is not correct.  spi_write_then_read() which returns zero on success.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Sergey Yanovich <ynvich@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>