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: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NDean Luick <dean.luick@intel.com>
Signed-off-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>