ibcore: 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.

Linux commit:
0f6ef65d1c6ec8deb5d0f11f86631ec4cfe8f22e

Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit ac4174e064f447ed3b19a8b395625b7d2b3739d1)