staging: comedi: ni_tiocmd: tidy up ni_tio_input_cmd()

The cmd->start_src is validated in the (*do_cmdtest) before this
function is called. All valid trigger sources are handled so the
default BUG() case can never occure.

For aesthetics, refactor the switch into if/else tests and remove
the BUG().

For aesthetics, rename the local variable 'retval' to simply 'ret'.

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>