hci_usb: Fix incorrect length log

Fixes following warning:
...
warning: format '%u' expects argument of type 'unsigned int', but
argument 6 has type 'int32_t * {aka int *}' [-Wformat=]
...

Change-Id: I3ac523b7c8daf08b0c8e7b43aad270306fe5d744
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>