API: Improve glue mid-layer of the API demo application.

- Extend ub_dev_read() and ub_dev_recv() so they return the length actually
read, which allows for better control and error handling (this introduces
additional error code API_ESYSC returned by the glue mid-layer).

- Clean up definitions naming and usage.

- Other minor cosmetics.

Note these changes do not touch the API proper, so the interface between
U-Boot and standalone applications remains unchanged.

Signed-off-by: Rafal Jaworowski <raj@semihalf.com>

include/api_public.h

@@ -57,6 +57,7 @@
 #define API_ENOMEM		3	/* no memory		*/
 #define API_EBUSY		4	/* busy, occupied etc.	*/
 #define API_EIO			5	/* I/O error		*/
+#define API_ESYSC		6	/* syscall error	*/
 
 typedef	int (*scp_t)(int, int *, ...);