USB: cxacru: check data length is not negative

When attempting to read data that is not actually
an array of values, the length may be negative
which causes an Oops due to a likely access off
the end of the data array.

This bug should not occur under normal use unless
the device returns an invalid response.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>