rtc: ds1374: Remove unused variable

Remove unused variable 'res' and fix the following build warning:

drivers/rtc/rtc-ds1374.c:667:6: warning: unused variable 'res' [-Wunused-variable]

Reported-by: Olof's autobuilder <build@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>