USB: cxacru: potential underflow in cxacru_cm_get_array()

commit 2a0ebf80aa95cc758d4725f74a7016e992606a39 upstream.

The value of "offd" comes off the instance->rcv_buf[] and we used it as
the offset into an array.  The problem is that we check the upper bound
but not for negative values.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>