staging: comedi: adl_pci9111: rename local var in pci9111_ai_do_cmd_test()

For aesthetics, rename the local variable 'error' to 'err'. This is the
normal name for this local variable in (*do_cmdtest) functions.

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>