greybus: fix unbalanced mutex

Running "make coccicheck" on the Greybus code reports that
gb_mmc_get_ro() and gb_mmc_get_cd() can return without releasing
the mutex it acquired if there's an error.  Fix this.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>