staging/rdma/hfi1: Consider VL15 MTU also when calculating the maximum VL MTU

Currently, only MTUs of VLs 0-7 are checked when calculating the maximum VL
MTU which is used to set the port MTU capability in DCC_CFG_PORT_CONFIG CSR
This can cause a port MTU capability to be set to 0 if MTUs of VLs 0-7 is 0
This would affect the VL15 traffic.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Reviewed-by: Arthur Kepner <arthur.kepner@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>