hw/arm/stellaris: Fix gptm_write() error message

Missed in df3692e0.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180624040609.17572-16-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>