remoteproc: imx_rproc: Slightly simplify code in 'imx_rproc_probe()'

We can return directly at the beginning of the function and save the 'err'
label.
We can also explicitly return 0 when the probe succeed.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>