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/[e-mail]
[2] https://lore.kernel.org/r/[e-mail]
[3] https://lore.kernel.org/r/[e-mail]
[4] https://lore.kernel.org/r/[e-mail]
Cc: Liam Beguin <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: [e-mail] # v5.8+
Signed-off-by: Gao Xiang <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

arch/parisc/include/asm/cmpxchg.h

···
 #endif
 	case 4: return __cmpxchg_u32((unsigned int *)ptr,
 				     (unsigned int)old, (unsigned int)new_);
-	case 1: return __cmpxchg_u8((u8 *)ptr, (u8)old, (u8)new_);
+	case 1: return __cmpxchg_u8((u8 *)ptr, old & 0xff, new_ & 0xff);
 	}
 	__cmpxchg_called_with_bad_pointer();
 	return old;