umbim: add support for wwan device class

umbim: add support for wwan device class

Some MBIM devices can exist on an MHI bus (over PCIe) instead of being presented as USB devices.

In such cases the interface name lookup needs to be done from /sys/class/wwan/ instead of /sys/class/usbmisc/

Add another readlink call in case the first lookup fails.

This allows the MBIM protocol to find the interface name and then work with both type of devices provided that /etc/config/network has the right device for MBIM interface (such as /dev/wwan0mbim0 in case of MHI)

Signed-off-by: Antti Seppälä <a.seppala@gmail.com>