remoteproc: core: Ensure error message is clear

Before this patch, the dma_alloc_coherent() failure path printed out:

  "dma_alloc_coherent err: 16760832"

... alluding to the Linux error code being 16760832, but seeing as
Linux error codes are all negative, this looks like a signed/unsigned
issue.  In fact, the message is trying to print the length of the
requested memory region.  Let's clear that up.

While we're at it, let's standardise the way 'len' is printed.  In
all other locations 'len' is in hex prefixed by a '0x' for clarity.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>