rtc: cpcap: kfreeing devm allocated memory

We shouldn't kfree(rtc) because is devm_ managed memory.  It leads to a
double free.

Fixes: dd3bf50b ("rtc: cpcap: new rtc driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>