bus: mhi: pci_generic: Increase num of elements in hw event ring

We met some sporadic modem crashes during high throughput testing, this
has been root caused to a lack of elements in the event ring. Indeed,
the modem is simply crashing when event ring becomes empty.

It appears that the total number event ring elements is too low given
the performances of the modem (IPA hardware accelerator). This change
increases the number of elements in the hardware event ring to 2048,
which is aligned with what is defined in downstream version:
https://source.codeaurora.org/quic/la/kernel/msm-4.14/tree/arch/arm64/boot/dts/qcom/sm8150-mhi.dtsi?h=msm-4.14#n482

With this change, modem coes not crash anymore.

Note: An event ring element is 16-Byte, so the total memory usage of
a hardware event ring is now 32KB.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1612514195-8257-1-git-send-email-loic.poulain@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>