reset: remove remaining WARN_ON() in <linux/reset.h>

Commit bb475230 ("reset: make optional functions really optional")
gave a new meaning to _get_optional variants.

The differentiation by WARN_ON() is not needed any more.  We already
have inconsistency about this; (devm_)reset_control_get_exclusive()
has WARN_ON() check, but of_reset_control_get_exclusive() does not.

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