Change USB strings handling code

This commit add an extra field to the _usbd_device, that allows to
keep track of the number of USB strings  which allows simplify
boundaries checking code in usb_standard_get_descriptor.

This commit also changes the index base for strings in
usb_standard_get_descriptor which allows to get rid of necessity to
have a dummy one-character string in a strings array.

lib/usb/usb_private.h

@@ -29,6 +29,7 @@ extern struct _usbd_device {
 	const struct usb_device_descriptor *desc;
 	const struct usb_config_descriptor *config;
 	const char **strings;
+	int num_strings;
 
 	u8 *ctrl_buf;  /**< Internal buffer used for control transfers */
 	u16 ctrl_buf_len;