libertas: Fix lbs_prb_rsp_limit_set()

The kstrtoul() test was reversed so this always returned -ENOTSUPP.

Fixes: 27d7f477 ("net: wireless: replace strict_strtoul() with kstrtoul()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: James Cameron <quozl@laptop.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>