net: usbnet: Move IPC log to module init/exit

We want to keep the IPC log preserved even if the interface
disconnect happens. To achieve that, make IPC log a global
structure and move initialization from probe to module init.

Change-Id: I1d98529c4fbd0bddb3efccb3d3f1d0058d2ad3e2
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>