staging: rtl8188eu: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/latest/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/20200707182008.GA341@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/core/rtw_mlme_ext.c

@@ -4132,7 +4132,7 @@ void mgt_dispatcher(struct adapter *padapter, struct recv_frame *precv_frame)
 			ptable->func = &OnAuth;
 		else
 			ptable->func = &OnAuthClient;
-		/* fall through */
+		fallthrough;
 	case WIFI_ASSOCREQ:
 	case WIFI_REASSOCREQ:
 	case WIFI_PROBEREQ:

drivers/staging/rtl8188eu/hal/usb_halinit.c

@@ -1728,7 +1728,7 @@ void rtw_hal_get_hwreg(struct adapter *Adapter, u8 variable, u8 *val)
 	switch (variable) {
 	case HW_VAR_BASIC_RATE:
 		*((u16 *)(val)) = Adapter->HalData->BasicRateSet;
-		/* fall through */
+		fallthrough;
 	case HW_VAR_TXPAUSE:
 		val[0] = usb_read8(Adapter, REG_TXPAUSE);
 		break;

drivers/staging/rtl8188eu/os_dep/usb_ops_linux.c

@@ -400,7 +400,7 @@ static void usb_read_port_complete(struct urb *purb, struct pt_regs *regs)
 		case -ENODEV:
 		case -ESHUTDOWN:
 			adapt->bSurpriseRemoved = true;
-			/* fall through */
+			fallthrough;
 		case -ENOENT:
 			adapt->bDriverStopped = true;
 			RT_TRACE(_module_hci_ops_os_c_, _drv_err_,