hw/misc/bcm2835_property: Reduce scope of variables in mbox push function

hw/misc/bcm2835_property: Reduce scope of variables in mbox push function

In bcm2835_property_mbox_push(), some variables are defined at function scope
but used only in a smaller scope of the function:
 * tag, bufsize, resplen are used only in the body of the while() loop
 * tmp is used only for RPI_FWREQ_SET_POWER_STATE (and is badly named)

Declare these variables in the scope where they're needed, so the code
is easier to read.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240723131029.1159908-5-peter.maydell@linaro.org