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/asix.c b/drivers/usb/eth/asix.c
index 3a8ec0b..448f524 100644
--- a/drivers/usb/eth/asix.c
+++ b/drivers/usb/eth/asix.c
@@ -7,6 +7,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <net.h>
 #include <usb.h>
 #include <malloc.h>
 #include <memalign.h>