USB: gadget: rndis: Fix NULL pointer crash during proc file read

Proc files are created during bootup and can be accessed before
cable connect or before enabling rndis. This could cause NULL
pointer access if read before cable connection or rndis enabled
as dev pointer is being accessed, which will be set during
set configuration request. Hence fix this by having check for
NULL pointer access.

Change-Id: I84d987e217e7493260e2dde8e3d1626249b8ac22
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>