rtc: sh: stop resetting time to epoch

There is no point in resetting the time to epoch as this means that
userspace will never get the valuable information that time is actually
invalid.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
[Anders Roxell <anders.roxell@linaro.org>: remove unused variable r]
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>