Quick tip on debugging the Inline::C stuff (in case you aren't already aware of it):  add NOCLEAN to the generated Makefile (after perl Makefile.PL, before make). I.e.

.pm.inl:
        $(PERL) -Mblib -MInline=NOISY,NOCLEAN,_INSTALL_ -MDevice::USB 
+-e1 0.31 $(INST_ARCHLIB)
                                      ^^^^^^^
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This way, the intermediately generated C code doesn't get cleaned up (in the _Inline subdir), so you can look at the respective wrapper functions — in this case (in USB.c)

XS(XS_Device__USB_libusb_get_descriptor)
{
    dXSARGS;
    if (items != 5)
        Perl_croak(aTHX_ "Usage: Device::USB::libusb_get_descriptor(de
+v, type, index, buf, size)");
    {
        void *  dev = INT2PTR(void *,SvIV(ST(0)));
        unsigned char   type = (unsigned char)SvUV(ST(1));
        unsigned char   index = (unsigned char)SvUV(ST(2));
        void *  buf = INT2PTR(void *,SvIV(ST(3)));          // <-- !!
        int     size = (int)SvIV(ST(4));
        int     RETVAL;
        dXSTARG;

        RETVAL = libusb_get_descriptor(dev, type, index, buf, size);
        XSprePUSH; PUSHi((IV)RETVAL);
    }
    XSRETURN(1);
}
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(which confirms BrowserUk's analysis)