reset: make device_reset_optional() really optional

Commit bb475230 ("reset: make optional functions really optional")
converted *_get_optional* functions, but device_reset_optional() was
left behind.  Convert it in the same way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>