staging: comedi: adl_pc19111: rename 'async_cmd' variable in (*do_cmd)

The comedi_cmd pointer is typically just called 'cmd'. Rename the local
variable in pci9111_ai_do_cmd().

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