RDS: IB: Initialize max_items based on underlying device attributes

Use max_1m_mrs/max_8k_mrs while setting max_items, as the former
variables are set based on the underlying device attributes.

Signed-off-by: Avinash Repaka <avinash.repaka@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>