Fix USB/FIQ lock-ups

What I did - no rocket science [1]:

* Applied RT-patches 4.19.72-rt26 on top of rpi-4.19.y / 4.19.79 (most recent
  version applying rt-patch properly)
* Applied a slightly rebased version of the original (4.14) fiq-patch [2]
* grepped for 'fiq_fsm_spin_lock(' and 'fiq_fsm_spin_unlock(' and added missing
  rt-specific replacements
* rebased changes back to rpi-4.19.y-rt

What this patch does:

* add one missing pair of fiq_fsm_spin_lock/fiq_fsm_spin_unlock replacements

With builds of [1] Rapsi3 is running without a singe issue for two weeks now
and it was stressed by

* moving gigabytes from USB-Stick to SDCard
* several usb-midi-keyboard jam sessions

Addresses [3]

[1] https://github.com/schnitzeltony/meta-raspi-light/tree/master/recipes-kernel/linux
[2] raspberrypi/linux@05dd5c4
[3] raspberrypi/linux#2943

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>

drivers/usb/host/dwc_otg/dwc_otg_cil_intr.c

@@ -1336,6 +1336,7 @@ static inline uint32_t dwc_otg_read_common_intr(dwc_otg_core_if_t * core_if, gin
 #ifdef CONFIG_USB_DWC_OTG_LPM
 	gintmsk_common.b.lpmtranrcvd = 1;
 #endif
+	unsigned long flags;
 	gintmsk_common.b.restoredone = 1;
 	if(dwc_otg_is_device_mode(core_if))
 	{
@@ -1345,17 +1346,15 @@ static inline uint32_t dwc_otg_read_common_intr(dwc_otg_core_if_t * core_if, gin
 		gintmsk_common.b.portintr = 1;
 	}
 	if(fiq_enable) {
-		local_fiq_disable();
-		fiq_fsm_spin_lock(&hcd->fiq_state->lock);
+		fiq_fsm_spin_lock_irqsave(&hcd->fiq_state->lock, flags);
 		gintsts.d32 = DWC_READ_REG32(&core_if->core_global_regs->gintsts);
 		gintmsk.d32 = DWC_READ_REG32(&core_if->core_global_regs->gintmsk);
 		/* Pull in the interrupts that the FIQ has masked */
 		gintmsk.d32 |= ~(hcd->fiq_state->gintmsk_saved.d32);
 		gintmsk.d32 |= gintmsk_common.d32;
 		/* for the upstairs function to reenable - have to read it here in case FIQ triggers again */
 		reenable_gintmsk->d32 = gintmsk.d32;
-		fiq_fsm_spin_unlock(&hcd->fiq_state->lock);
-		local_fiq_enable();
+		fiq_fsm_spin_unlock_irqrestore(&hcd->fiq_state->lock, flags);
 	} else {
 		gintsts.d32 = DWC_READ_REG32(&core_if->core_global_regs->gintsts);
 		gintmsk.d32 = DWC_READ_REG32(&core_if->core_global_regs->gintmsk);