ide-cd: merge cdrom_start_read() and cdrom_start_write()

Add handling of read requests to cdrom_start_write(), rename it
to cdrom_start_rw() and remove no longer needed cdrom_start_read().

There should be no functionality changes caused by this patch.

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