IB/hfi1: Add functions to parse BTH/IB headers

Improve code readablity by adding inline functions
to read specific BTH/IB fields without knowledge of
byte offsets.

Reviewed-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>