mhi: pci_generic: Set irq moderation value to 1ms for hw channels

MHI hardware channels are usually the hardware accelerated data path
e.g. IP packets path for modems. This path needs to be optimized for
low latency and high throughput. After several tests on FN980m SDX55
based modem, it seems 1ms is a good default irq_moderation value:

- It allows to reach the maximum download throughput
- It introduces limited latency (5ms is too high)
- It prevents interrupt flooding

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>