ide-cd: simplify request issuing path

Call cdrom_start_packet_command() only from the ->do_request() routine.
As a nice side effect, this improves code readability a bit.

There should be no functionality change resulting from this patch.

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