kbuild: Move -Wenum-enum-conversion to W=2

-Wenum-enum-conversion was strengthened in clang-19 to warn for C, which
caused the kernel to move it to W=1 in commit 75b5ab134bb5 ("kbuild:
Move -Wenum-{compare-conditional,enum-conversion} into W=1") because
there were numerous instances that would break builds with -Werror.
Unfortunately, this is not a full solution, as more and more developers,
subsystems, and distributors are building with W=1 as well, so they
continue to see the numerous instances of this warning.

Since the move to W=1, there have not been many new instances that have
appeared through various build reports and the ones that have appeared
seem to be following similar existing patterns, suggesting that most
instances of this warning will not be real issues. The only alternatives
for silencing this warning are adding casts (which is generally seen as
an ugly practice) or refactoring the enums to macro defines or a unified
enum (which may be undesirable because of type safety in other parts of
the code).

Move the warning to W=2, where warnings that occur frequently but may be
relevant should reside.

Cc: stable@vger.kernel.org
Fixes: 75b5ab134bb5 ("kbuild: Move -Wenum-{compare-conditional,enum-conversion} into W=1")
Link: https://lore.kernel.org/ZwRA9SOcOjjLJcpi@google.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>