include/rdma/opa_addr.h: Fix an endianness issue

IB_MULTICAST_LID_BASE is defined as follows:

  #define IB_MULTICAST_LID_BASE   cpu_to_be16(0xC000)

Hence use be16_to_cpu() to convert it to CPU endianness. Compile-tested
only.

Fixes: af808ece ("IB/SA: Check dlid before SA agent queries for ClassPortInfo")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Cc: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>