staging: comedi: ni_tio: remove BUG() in ni_tio_set_gate_src()

This BUG() can never happen. The previous ni_tio_has_gate2_registers()
check will have already caused the function to return -EINVAL.

Signed-off-by: H Hartley Sweeten <[e-mail]>
Reviewed-by: Ian Abbott <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/comedi/drivers/ni_tio.c

···
 		case ni_gpct_variant_660x:
 			return ni_660x_set_gate2(counter, src);
 		default:
-			BUG();
-			break;
+			return -EINVAL;
 		}
 		break;
 	default: