IB/hfi1: Fix potential buffer overflow

This fixes potential buffer overflow because the sprintf function
doesn't check buffer boundaries. Use snprintf instead.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>