modem/msm-modem: support mbim

I have a device (lenovo-21bx / x13s) with a modem, and boot takes 45 seconds longer because it does not have any of the other /dev/wwan* types listed, and it shows many node with id 0 not found in QRTR bus messages because it's not able to use the QRTR fallback. By opening the /dev/wwan0mbim0 device using this patch, it can at least find the modem and get status from it, but I haven't tested that it works with a SIM installed. With this patch, I now experience the 4 second SIM timeout, which is far less annoying than 45s.