hd: clean up request completion paths

hd read/write_intr() functions manually manipulate request to
incrementally complete it, which block layer already supports.  Simply
use block layer completion routines instead of manual partial
completion.

While at it, clear unnecessary elv_next_request() check at the tail of
read_intr().  This also makes read and write_intr() more consistent.

[ Impact: cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>