IB/hfi1: Remove incorrect IS_ERR check

Remove IS_ERR check from caching code as the function being called does
not actually return error pointers.

Fixes: f19bd643

: "IB/hfi1: Prevent NULL pointer deferences in caching code"
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>