usb: Fix build for 64 bit platforms

Use proper format for size_t type eliminating warnings of type:

...
error: format '%u' expects argument of type 'unsigned int', but
argument 2 has type 'size_t' {aka 'long unsigned int'}
[-Werror=format=]
...

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