net: usb: ax88179_178a: Fix handling of IP header alignment

Asix mini-driver passes incorrect skb->len to usbnet due to
incorrect handling of IP alignment pseudo-header.
Fix updates to skb->len by correctly performing skb_pull by
NET_IP_ALIGN bytes.

Change-Id: I92ec4631875fd3ab7d1f20c947084ab2fa187555
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>