rtc: ep93xx: Fix 'rtc' may be used uninitialized warning

rtc: ep93xx: Fix 'rtc' may be used uninitialized warning

commit 92d921c5d "rtc: ep93xx: Initialize drvdata before registering device"
ensures the drvdata is initialized prior to registering the rtc device.
But it set the drvdata to an uninitialized pointer.
Thus calling platform_get_drvdata in ep93xx_rtc_remove does not get correct address.

This patch fixes below warning by adding struct rtc_device *rtc to struct ep93xx_rtc.
Then set platform drvdata to ep93xx_rtc instead of rtc.

  CC      drivers/rtc/rtc-ep93xx.o
drivers/rtc/rtc-ep93xx.c: In function 'ep93xx_rtc_probe':
drivers/rtc/rtc-ep93xx.c:154: warning: 'rtc' may be used uninitialized in this function

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>