rpmsg: use local 'dev' variable

'&rpdev->dev' is already cached as local variable, so use it to simplify
the code.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220429195946.1061725-3-krzysztof.kozlowski@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>