rtc: ds1347: simplify getting .driver_data

Get 'driver_data' from 'struct device' directly. Going via spi_device is an
unnecessary step.

Link: https://lore.kernel.org/r/20191007134724.15505-4-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>