linux-user/arm: Fix return value of SYS_cacheflush

linux-user/arm: Fix return value of SYS_cacheflush

Although the emulated cacheflush syscall does nothing, it still needs to
return zero to indicate success.

Cc: qemu-stable@nongnu.org
Signed-off-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Message-id: 20250613-cache-v1-1-ee9f4a9ba81b@gmx.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>