usb: atm: remove unnecessary code

'index' is an unsigned variable, and less-than-zero comparison of an unsigned
variable is never true.

Addresses-Coverity-ID: 115396
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>