pds_core: fix mutex double unlock in error path

Fix a double unlock in an error handling path by unlocking as soon as
the error is seen and removing unlocks in the error cleanup path.

Link: https://lore.kernel.org/kernel-janitors/209a09f6-5ec6-40c7-a5ec-6260d8f54d25@kili.mountain/
Fixes: 523847df

 ("pds_core: add devcmd device interfaces")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>