Driver includes magic numbers. Defining constants or using existing
constants aids the readability of the code.

Magic number '12' is used for two ethernet addresses (6 bytes
each). ETH_ALEN is already defined within the kernel to 6. We can us
the expression '2 * ETH_ALEN' to make this code explicit.

Magic number '20' refers to the data size, in bytes, of a struct
ether_hdr (described in eap_packet.h). We can define a constant for
this purpose, making the code explicit and easier to read.

Define constant. Remove magic numbers, using newly defined constant
and/or expression using existing kernel constant.
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>