remove ethernet.h header file

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

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.

Link: https://lore.kernel.org/r/20210824123621.10801-4-straube.linux@gmail.com

include/rtw_recv.h

@@ -89,6 +89,8 @@
 extern u8 rtw_bridge_tunnel_header[];
 extern u8 rtw_rfc1042_header[];
 
+#define LLC_HEADER_SIZE 6
+
 /* for Rx reordering buffer control */
 struct recv_reorder_ctrl {
 	_adapter	*padapter;