RDMA/core: Do not expose unsupported counters

If the provider driver (such as rdma_rxe) doesn't support pma counters,
avoid exposing its directory similar to optional hw_counters directory.
If core fails to read the PMA counter, return an error so that user can
retry later if needed.

Fixes: 35c4cbb1

 ("IB/core: Create get_perf_mad function in sysfs.c")
Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>