cdrom: don't attempt to fiddle with cdo->capability
We can't modify cdo->capability as it is defined as a const.
Change the modification hack to just WARN_ON_ONCE() if we hit
any of the invalid combinations.
This fixes a regression for pcd, which doesn't work after the
constify patch.
Fixes: 853fe1bf7554 ("cdrom: Make device operations read-only") Tested-by: Ondrej Zary <linux@rainbow-software.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>