staging/rdma/hfi1: Fix snoop packet length calculation

The LRH has a 12 bit packet length field, not 11 bit. This caused a
snoop packet length miscalculation leading to a crash when sending a
large ping over IPoIB while running opapacketcapture.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>