libata: update ATAPI overflow draining

For misc ATAPI commands which transfer variable length data to the
host, overflow can occur due to application or hardware bug.  Such
overflows can be ignored safely as long as overflow data is properly
drained.  libata HSM implementation has this implemented in
__atapi_pio_bytes() and recently updated for 2.6.24-rc but it requires
further improvements.  Improve drain logic such that...

* Report overflow errors using ehi desc mechanism instead of printing
  directly.

* Properly calculate the number of bytes to be drained considering
  actual number of consumed bytes for partial draining.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>