dwc_otg: more robust interrupt handling

This PR fixes issues such as the error message
```
ERROR::handle_hc_chhltd_intr_dma:2537: handle_hc_chhltd_intr_dma: Channel 7, DMA Mode -- ChHltd set, but reason for halting is unknown, hcint 0x00000002, intsts 0x04400001
```
popping up in the log since
https://github.com/raspberrypi/linux/commit/db4fad7380c83b6e1b62dfb3fd61e55d031a04fc. 
I would prefer a round of testing by people that have seen this issue before merging for general consumption, I will advertise as necessary in the relevant places.