staging: comedi: dt3000: Reformat multiple line dereference

Reformat multiple line dereferences for &cmd->scan_begin_arg. Problem
detected by checkpatch.

Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Link: https://lore.kernel.org/r/20200314135852.GA6336@deeUbuntu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>