remoteproc: imx_dsp_rproc: Simplify start/stop error handling

remoteproc: imx_dsp_rproc: Simplify start/stop error handling

Replace goto-based error handling with early return pattern in
imx_dsp_rproc_{start,stop}() functions, and simplify if-else logic.

No functional changes, only code structure improvements for better
maintainability.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20251119-imx-dsp-2025-11-19-v4-12-adafd342d07b@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>