staging: comedi: ni_labpc: don't pass struct comedi_cmd by value

`labpc_suggest_transfer_size()` has a parameter of type `struct
comedi_cmd` passed by value.  Change it to pass by pointer reference.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>