staging: comedi: rearrange comedi_write() code

Rearrange the code in `comedi_write()` to reduce the amount of
indentation.  The code never reiterates the `while` loop once `count`
has become non-zero, so we can check that in the `while` condition to
save an indentation level.  (Note that `nbytes` has been checked to be
non-zero before entering the loop, so we can remove that check.)  Move
the code that makes the subdevice "become non-busy" outside the `while`
loop, using a new flag variable `become_nonbusy` to decide whether it
needs to be done.  This simplifies the wait queue handling so there is a
single place where the task is removed from the wait queue, and we can
remove the `on_wait_queue` flag variable.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>