staging: ks7010: change ETHER_HDR_SIZE definition to use the size of struct ether_hdr

This commit changes ETHER_HDR_SIZE definition which is a hardcoded
number using the size of the structure ether_hdr which is the one
for what this definition has been created. This improves readability
and makes clear the intention of this definition along the code.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>