IB/hfi1: Use 4096 for default active MTU in query_qp

Currently, if a port is queried that has an invalid
Maximum Transmission Unit, driver reports default MTU of 2048.
This in incorrect.

Use default value of 4096 if invalid.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>