usb: gadget: u_ether: Changes to enable skb timestamping

Timestamp is recorded for the exit of a packet,
and just after the driver delivers the packet to
network stack. This feature is disabled by default.

To enable timestamping use this command:
echo 1 > /sys/module/g_android/parameters/skb_timestamp_enable

Change-Id: I1022fd3021699d07460054984bf95f2bbef9f29d
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>
Signed-off-by: Sai Krishna Juturi <jsaikrishna@codeaurora.org>