remoteproc: Fix an error code in devm_rproc_alloc()

The comments say that this function should return NULL on error and the
caller expects NULL returns as well so I have modified the code to match.
Returning an ERR_PTR(-ENOMEM) would lead to an OOps.

Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Fixes: 305ac5a7

 ("remoteproc: Add device-managed variants of rproc_alloc/rproc_add")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200520120705.GH172354@mwanda
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>