net: usb: rmnet_usb_ctrl: Fix use after free issue

In rmnet_usb_ctrl_init dev is freed again it is
dereferenced with in same block. Fix this issue
by taking derefernced value into a local variable
prior to freeing dev.

Change-Id: Ic51accc8949e97c8aaeda558adcc9d404fb2a26d
Signed-off-by: Sai Krishna Juturi <jsaikrishna@codeaurora.org>