rtc: Fix ioctl error path return

Bryan Henderson noticed that the  "RTC: Fix rtc driver ioctl specific
shortcutting" commit has a small bug: When an ioctl is called with an
invalid command code and the clock driver does not have an "ioctl"
method, the ioctl returns rc 0 instead of -ENOTTY.

This patch fixes the issue.

CC: Bryan Henderson <bryanh@giraffe-data.com>
CC: Gabor Z. Papp <gzp@papp.hu>
Reported-by: Bryan Henderson <bryanh@giraffe-data.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>