ide-cd: move request prep from cdrom_start_rw_cont to rq issue path

... by factoring out the rq preparation code into a separate
function called in the request routine.  As a nice side effect,
this minimizes the IRQ handler execution time.

There should be no functionality change resulting from this patch.

[bart: s/HWGROUP()/drive->hwif->hwgroup/ and remove extra newline]

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