ide-cd: add ide_cd_drain_data() helper

Add ide_cd_drain_data() and use it in cdrom_{buffer_sectors,read_intr}()
(as a nice side-effect this cuts 0.5kB of code from ide-cd.o).

There should be no functionality changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>