usb: phy: Use fallthrough pseudo-keyword

Replace /* FALLTHROUGH */ comment with pseudo-keyword macro fallthrough[1]

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughAcked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210505141936.4343-1-jj251510319013@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/phy/phy-isp1301-omap.c

@@ -555,7 +555,7 @@ static void otg_update_isp(struct isp1301 *isp)
 	case OTG_STATE_A_PERIPHERAL:
 		if (otg_ctrl & OTG_PULLUP)
 			goto pullup;
-		/* FALLTHROUGH */
+		fallthrough;
 	// case OTG_STATE_B_WAIT_ACON:
 	default:
 pulldown: