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/net/smc911x.h b/drivers/net/smc911x.h
index ce66900..0a27f3c 100644
--- a/drivers/net/smc911x.h
+++ b/drivers/net/smc911x.h
@@ -8,6 +8,8 @@
 #ifndef _SMC911X_H_
 #define _SMC911X_H_
 
+#include <net.h>
+
 /* Below are the register offsets and bit definitions
  * of the Lan911x memory space
  */