ide-cd: merge cdrom_start_read_continuation() and cdrom_start_rw_cont()

* Add handling of write requests to cdrom_start_read_continuation(),
  rename it to cdrom_start_rw_cont() and remove no longer needed
  cdrom_start_write_cont().

* Remove redundant '(rq->sector & (sectors_per_frame - 1)' check.

There should be no functionality changes caused by this patch.

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