parisc: avoid a warning on u8 cast for cmpxchg on u8 pointers

parisc: avoid a warning on u8 cast for cmpxchg on u8 pointers

commit b344d6a83d01 ("parisc: add support for cmpxchg on u8 pointers")
can generate a sparse warning ("cast truncates bits from constant
value"), which has been reported several times [1] [2] [3].

The original code worked as expected, but anyway, let silence such
sparse warning as what others did [4].

[1] https://lore.kernel.org/r/202104061220.nRMBwCXw-lkp@intel.com
[2] https://lore.kernel.org/r/202012291914.T5Agcn99-lkp@intel.com
[3] https://lore.kernel.org/r/202008210829.KVwn7Xeh%25lkp@intel.com
[4] https://lore.kernel.org/r/20210315131512.133720-2-jacopo+renesas@jmondi.org
Cc: Liam Beguin <liambeguin@gmail.com>
Cc: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.8+
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>