reset: core: fix reset_control_put

Commit "reset: make optional functions really optional" missed to
adjust one check in reset_control_put, causing a NULL pointer
access for optional resets.

Fixes: bb475230 "reset: make optional functions really optional"
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>