Fix misc spelling errors found by lintian

Signed-off-by: Loïc Minier <loic.minier@linaro.org>
diff --git a/common/cmd_usb.c b/common/cmd_usb.c
index b5731a7..3ba6fff 100644
--- a/common/cmd_usb.c
+++ b/common/cmd_usb.c
@@ -588,7 +588,7 @@
 					break;
 			}
 			if (dev == NULL) {
-				printf("*** NO Device avaiable ***\n");
+				printf("*** No device available ***\n");
 				return 0;
 			} else {
 				usb_display_desc(dev);
diff --git a/common/usb_storage.c b/common/usb_storage.c
index 6963e6d..9ecf165 100644
--- a/common/usb_storage.c
+++ b/common/usb_storage.c
@@ -254,7 +254,7 @@
 		dev = usb_get_dev_index(i); /* get device */
 		USB_STOR_PRINTF("i=%d\n", i);
 		if (dev == NULL)
-			break; /* no more devices avaiable */
+			break; /* no more devices available */
 
 		if (usb_storage_probe(dev, 0, &usb_stor[usb_max_devs])) {
 			/* OK, it's a storage device.  Iterate over its LUNs