usb: net: gadget: Remove support for other kernel versions

The updated ax88179_178a driver segregates between different
kernel versions. Clean up the code to retain support for
kernel version 3.18.

Change-Id: I9a1051c5747134885ca1abe12265db2f889c3873
Signed-off-by: Sagar Sahu <sahusag@codeaurora.org>