staging: comedi: gsc_hpdi: cleanup step 3 of me4000_ai_do_cmd_test()

Remove the dev_err() noise and convert the error handling to the
normal form (err |= -EINVAL) used in the do_cmdtest functions.

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