staging: et131x: Use skb_headlen() where appropriate

(skb->len - skb->data_len) is used in several places in the et131x
driver code. Converted all instances of this to use skb_headlen()
which is more readable.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>