Impact: cleanup

Byte size is what most issue functions deal with, make
idetape_queue_rw_tail() and its wrappers take byte size instead of
sector counts.  idetape_chrdev_read() and write() functions are
converted to use tape->buffer_size instead of ctl from tape->cap.

This cleans up code a little bit and will ease the next r/w
reimplementation.
Signed-off-by: Tejun Heo <tj@kernel.org>