usb: gadget: u_ether: Add NULL check for IN and OUT ep context

u_ether driver can used for a function driver which has only
IN endpoint for data transfer. This results into OUT ep context
as NULL and causes NULL pointer dereference. Hence check
both IN and OUT ep context before dereferencing the context pointer.

Change-Id: I77d909ce0954f3cc87fc2bd2be254854e4662d4a
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>