staging: comedi: s526: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200727184247.GA29207@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/s526.c

@@ -400,7 +400,7 @@ static int s526_gpct_winsn(struct comedi_device *dev,
 		if ((data[1] <= data[0]) || !data[0])
 			return -EINVAL;
 		/* to write the PULSE_WIDTH */
-		/* fall through */
+		fallthrough;
 	case INSN_CONFIG_GPCT_QUADRATURE_ENCODER:
 	case INSN_CONFIG_GPCT_SINGLE_PULSE_GENERATOR:
 		s526_gpct_write(dev, chan, data[0]);