staging: comedi: rtd520: Resolve multiline dereference

Reformat multi-line dereferencing of function arguments
&cmd->scan_begin_arg. Problem detected by checkpatch script.

Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Link: https://lore.kernel.org/r/20200314214154.GA3904@deeUbuntu


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