usb: gadget: u_ether: Add NULL check in helper APIs

APIs being called as part of configfs attributes require
to have NULL check for network device pointer. As user
can call show and store call back even if network device
is not allocated which results in to NULL pointer dereference.

For example: In non-NCM composition user tries
cat config/usb_gadget/g1/functions/ncm.0/ifname

Change-Id: Ia4a1c6bec6d20e41886c952b0365419f2a8effcb
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>