GCC4.6: Convert various empty macros to inline functions

Fix the following gcc4.6 problems:

cmd_date.c: In function ‘do_date’:
cmd_date.c:50:6: warning: variable ‘old_bus’ set but not used
[-Wunused-but-set-variable]
asix.c: In function ‘asix_init’:
asix.c:317:6: warning: variable ‘rx_ctl’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_parse_config’:
usb.c:331:17: warning: variable ‘ch’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_hub_port_connect_change’:
usb.c:1123:29: warning: variable ‘portchange’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_hub_configure’:
usb.c:1183:25: warning: variable ‘hubsts’ set but not used
[-Wunused-but-set-variable]
usb_storage.c: In function ‘usb_stor_CB_reset’:
usb_storage.c:466:6: warning: variable ‘result’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
diff --git a/include/i2c.h b/include/i2c.h
index 8ceb4c8..323150d 100644
--- a/include/i2c.h
+++ b/include/i2c.h
@@ -55,7 +55,10 @@
 #else
 #define CONFIG_SYS_MAX_I2C_BUS		1
 #define I2C_GET_BUS()		0
-#define I2C_SET_BUS(a)
+static inline int I2C_SET_BUS(unsigned int bus)
+{
+	return 0;
+}
 #endif
 
 /* define the I2C bus number for RTC and DTT if not already done */