staging: comedi: s626: use a local var for the s->async in s626_irq_handler()

The comedi_async pointer is used a number of times in this function.
For aesthetic reasons, use a local variable for the pointer.

Also, since was already have a pointer to the "cmd" use that in the
for loop instead of s->async->cmd (or even async->cmd).

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