memory: renesas-rpc-if: simplify with PTR_ERR_OR_ZERO

Use PTR_ERR_OR_ZERO to make the code a little bit simpler.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200724074038.5597-17-krzk@kernel.org