sentelic - release mutex upon register write failure

Make sure that mutex is released upon register writing failure.
This fixes boot freezing observed on ARM based OLPC
(http://dev.laptop.org/ticket/11357).

Signed-off-by: Paul Fox <pgf@laptop.org>
Signed-off-by: Tai-hwa Liang <avatar@sentelic.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
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