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] https://github.com/raspberrypi/linux/commit/05dd5c4368b56661786da5822d745818d8545d58
[3] https://github.com/raspberrypi/linux/issues/2943



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