net: qualcomm: rmnet: Use skb_put_zero() to simplify code

Use skb_put_zero() instead of hand-writing it. This saves a few lines of
code and is more readable.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>