pci-ftpci100: race condition in masking/unmasking interrupts

[Pali Rohár]

Hello!

I do not see any entry in MAINTAINERS file for pci-ftpci100.c driver, so
I'm not sure to whom should I address this issue...

During pci-aardvark review, Marc pointed one issue which is currently
available also in pci-ftpci100.c driver.

When masking or unmasking interrupts there is read-modify-write sequence
for FARADAY_PCI_CTRL2 register without any locking and is not atomic:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/pci-ftpci100.c?h=v5.13#n270

So there is race condition when masking/unmasking more interrupts at the
same time.