rtc: rtc-rs5c313: Drop obsolete platform_set_drvdata() call

Commit 284e2fa1 ("rtc: rtc-rs5c313: use
devm_rtc_device_register()"), removed the last user of the
driver-specific data.  Hence there is no longer a need to set it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200814110731.29029-2-geert+renesas@glider.be