net: rmnet_usb: Support multiple instances

Currently rmnet_usb supports a maximum of 4 net interfaces on a
single device at a time. This change allows the possibility of
multiple concurrent USB devices with RmNet interfaces to be
supported. The number and names of devices supported are now
specified by the following new module parameters:

* no_rmnet_devs - default: 1
* no_rmnet_insts_per_dev - default: 4
* rmnet_dev_names - default: "hsicctl"
* rmnet_data_init - initializes device nodes with above params

Userspace must now write to "rmnet_data_init" in order for the
control device nodes to be created.

Change-Id: I1e36d85b7fab4290ef64ef635162eb6e4de84e50
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>