usb-bulk-transfer: Clarify sign-iy-ness of param

Parameter is implicily converted to unsigned when assigned to the struct,
so make sure users are aware of this.

Test: compiled
Change-Id: I25ca6b24591497aa4e7ce8db262ce1099c2b7b09
diff --git a/libusbhost/usbhost.c b/libusbhost/usbhost.c
index 299fdc4..68aca17 100644
--- a/libusbhost/usbhost.c
+++ b/libusbhost/usbhost.c
@@ -600,7 +600,7 @@
 int usb_device_bulk_transfer(struct usb_device *device,
                             int endpoint,
                             void* buffer,
-                            int length,
+                            unsigned int length,
                             unsigned int timeout)
 {
     struct usbdevfs_bulktransfer  ctrl;