ide-cd: merge cdrom_read_intr() and cdrom_write_intr()

Add handling of read requests to cdrom_write_intr(), rename it
to cdrom_rw_intr() and remove no longer needed cdrom_read_intr().

There should be no functionality changes caused by this patch.

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