IB/hfi1: Use kernel default llseek for ui device

The ui device llseek had a mistake with SEEK_END and did
not fully follow seek semantics.  Correct all this by
using a kernel supplied function for fixed size devices.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>