ide-cd: prepare cdrom_rw_intr() and cdrom_newpc_intr() to be merged

In cdrom_newpc_intr():
* cleanup variables in the 'transfer data' loop

In cdrom_rw_intr():
* rename 'sectors_to_transfer' to 'thislen'
* rename 'this_transfer' to 'blen'
* keep number of bytes (instead of sectors) in 'thislen' and 'blen'
* call 'xferfunc' only once for 'blen'
* cache 'rq->buffer' in 'ptr' variable
* check for 'rq->bio' before setting 'ptr' and 'blen'
* check for 'ptr' instead of 'rq->current_nr_sectors'

There should be no functionality changes caused by this patch.

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