crypto: qat - prevent underflow in rp2srv_store()

The "ring" variable has an upper bounds check but nothing checks for
negatives.  This code uses kstrtouint() already and it was obviously
intended to be declared as unsigned int.  Make it so.

Fixes: dbc8876d ("crypto: qat - add rp2svc sysfs attribute")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>