bnxt_en: get the reduced max_irqs by the ones used by RDMA

When getting the max rings supported, get the reduced max_irqs
by the ones used by RDMA.

If the number MSIX is the limiting factor, this bug may cause the
max ring count to be higher than it should be when RDMA driver is
loaded and may result in ring allocation failures.

Fixes: 30f52947

 ("bnxt_en: Do not modify max IRQ count after RDMA driver requests/frees IRQs.")
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>