net: rmnet_usb: qmap: reset skb tail when changing data pointer

data pointer is moved to accommodate mux header but as tail pointer
is not updated resulting in loss of data during ip forwarding. Avoid
data loss by resetting tail pointer when manipulating data pointer.

Change-Id: Ica8c8fd25672cdb4ae036454fdaae98f1d327b22
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>