common: Drop net.h from common header

Move this header out of the common header. Network support is used in
quite a few places but it still does not warrant blanket inclusion.

Note that this net.h header itself has quite a lot in it. It could be
split into the driver-mode support, functions, structures, checksumming,
etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/eth/usb_ether.c b/drivers/usb/eth/usb_ether.c
index 3aca9ac..d76e2ff 100644
--- a/drivers/usb/eth/usb_ether.c
+++ b/drivers/usb/eth/usb_ether.c
@@ -7,7 +7,9 @@
 #include <dm.h>
 #include <errno.h>
 #include <malloc.h>
+#include <net.h>
 #include <usb.h>
+#include <asm/cache.h>
 #include <dm/device-internal.h>
 
 #include "usb_ether.h"