Hi,
I had the same deadlock. I programmed a wrong QSPI image then the U-Boot gets stuck and HPS flash programmer could not access it anymore.
What I did is: Remove the QSPI card and power on the system. While the system is running plug on the QSPI card (ensure not to make any short circuits!!). This will ensure that u-boot is not running and the card has power. Then erase the card with quartus_hps.
Regards
MillerTheRipper
EDIT: I found a much better smooth way to do prevent the HPS from booting. Try:
quartus_hps --boot=18 --cable=1 --operation=pv uboot_w_dtb-mkpimage.bin