ide-atapi: switch to rq->resid_len

Now that we have rq->resid_len, use it to account partial completion
amount during the lifetime of an rq, decrementing it on each successful
transfer. As a result, get rid of now unused pc->xferred.

While at it, remove noisy debug call in ide_prep_sense.

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