remoteproc: imx_dsp_rproc: Improve exception handling in imx_dsp_rproc_mbox_alloc()

remoteproc: imx_dsp_rproc: Improve exception handling in imx_dsp_rproc_mbox_alloc()

The label “err_out” was used to jump to another pointer check
despite of the detail in the implementation of the function
“imx_dsp_rproc_mbox_alloc” that it was determined already
that the corresponding variable contained an error pointer
because of a failed call of the function “mbox_request_channel_byname”.

Thus perform the following adjustments:

1. Return directly after a call of the function
   “mbox_request_channel_byname” failed for the input parameter “tx”.

2. Use more appropriate labels instead.

3. Reorder jump targets at the end.

4. Omit a function call and three extra checks.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Link: https://lore.kernel.org/r/d0e18bb1-afc4-8b6f-bb1c-b74b3bad908e@web.de
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>