IB/ipoib: Prevent setting negative values to max_nonsrq_conn_qp

Don't allow negative values to max_nonsrq_conn_qp. There is no functional
impact on a negative value but it is logicically incorrect.

Fixes: 68e995a2 ("IPoIB/cm: Add connected mode support for devices without SRQs")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>