cxl: use -Werror only with CONFIG_PPC_WERROR

Some developers really like to have -Werror enabled for their code, as
it helps to ensure warning free code. Others don't want -Werror, as it
(for example) can cause problems when newer (or older) compilers have
different sets of warnings, or new warnings can appear just when turning
up the warning level (e.g., make W=1 or W=2). Thus, it seems prudent to
have the use of -Werror be configurable.

It so happens that cxl is only built on PowerPC, and PowerPC already
has a nice set of Kconfig options for this, under CONFIG_PPC_WERROR. So
let's use that, and the world is a happy place again! (Note that
PPC_WERROR defaults to =y, so the common case compile should still be
enforcing -Werror.)

Fixes: d3d73f4b ("cxl: Compile with -Werror")
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>