ocxl: do not use C++ style comments in uapi header

Linux kernel tolerates C++ style comments these days. Actually, the
SPDX License tags for .c files start with //.

On the other hand, uapi headers are written in more strict C, where
the C++ comment style is forbidden.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>