usb: gadget: f_rndis: reduce NETTX irq caused by free skb header

This patch reduce unecessary NETTX softirq call caused by
free skb header. You will see this softirq comes twice while
there is only one TX packet to be transmitted.

So using dev_kfree_skb() instead of dev_kfree_skb_any() to
avoid this problem.

Cc: David S. Miller <davem@davemloft.net>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Macpaul Lin <macpaul@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>