staging: r8188eu: remove ethernet.h header file

There are only two definitions from ethernet.h used in the driver.
Namely:

#define ETHERNET_HEADER_SIZE           14      /*  Ethernet Header Length */
#define LLC_HEADER_SIZE                6       /*  LLC Header Length */

Both are only used in the file core/rtw_recv.c. Replace the usage of
ETHERNET_HEADER_SIZE with ETH_HLEN, move the definition of LLC_HEADER_SIZE
into rtw_recv.h and remove the now unused ethernet.h header file.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210824123621.10801-4-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>