soc: mediatek: mtk-mutex: Replace max handles number with definition

Replace the magic number "10", defining the maximum number of supported
handles with a MTK_MUTEX_MAX_HANDLES definition.
While at it, also change the type for `id` from a signed integer to
a unsigned 8 bits integer to save some (small) memory footprint, as
this number is never higher than 10.

This cleanup brings no functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230222094253.23678-7-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>