hw/misc/bcm2835_property: Fix handling of FRAMEBUFFER_SET_PALETTE

hw/misc/bcm2835_property: Fix handling of FRAMEBUFFER_SET_PALETTE

The documentation of the "Set palette" mailbox property at
https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface#set-palette
says it has the form:

    Length: 24..1032
    Value:
        u32: offset: first palette index to set (0-255)
        u32: length: number of palette entries to set (1-256)
        u32...: RGBA palette values (offset to offset+length-1)

We get this wrong in a couple of ways:
 * we aren't checking the offset and length are in range, so the guest
   can make us spin for a long time by providing a large length
 * the bounds check on our loop is wrong: we should iterate through
   'length' palette entries, not 'length - offset' entries

Fix the loop to implement the bounds checks and get the loop
condition right. In the process, make the variables local to
this switch case, rather than function-global, so it's clearer
what type they are when reading the code.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240723131029.1159908-2-peter.maydell@linaro.org
(cherry picked from commit 0892fffc2abaadfb5d8b79bb0250ae1794862560)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: context fix due to lack of
 v9.0.0-1812-g5d5f1b60916a "hw/misc: Implement mailbox properties for customer OTP and device specific private keys"
 also remove now-unused local `n' variable which gets removed in the next change in this file,
 v9.0.0-2720-g32f1c201eedf "hw/misc/bcm2835_property: Avoid overflow in OTP access properties")