UPSTREAM: rpmsg: Constify local variable in field store macro

commit e5f89131 upstream.

Memory pointed by variable 'old' in field store macro is not modified,
so it can be made a pointer to const.

Bug: 295334746
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419113435.246203-12-krzysztof.kozlowski@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 5c0da718)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: If5610892601368e855ec1be57e01dbbe06ae046a