RDMA/efa: Count admin commands errors

Add a new stat that counts admin commands failures, which might help when
debugging different issues.

Link: https://lore.kernel.org/r/20200420062213.44577-4-galpress@amazon.com
Reviewed-by: Daniel Kranzdorf <dkkranzd@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>