cdx: Use mutex guard to simplify error handling

cdx: Use mutex guard to simplify error handling

Mutex guard allows to drop one goto/break in error handling and the
less expected code of assigning -EINVAL to unsigned size_t count
variable.

Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20260109-of-for-each-compatible-scoped-v3-7-c22fa2c0749a@oss.qualcomm.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>