staging: rtlwifi: silence underflow warning

I'm not totally certain that it's necessary to put an upper limit here.
I think it happens at lower levels.  But if we are going to do that then
we should have a lower bound as well.

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