staging/rdma/hfi1: Fix counter read for cp

A cp or cat of /sys/kernel/debug/hfi1/hfi1_0/port1counters
produces the following message:

hfi1 0000:81:00.0: hfi1_0: index not supported
hfi1 0000:81:00.0: hfi1_0: read_cntrs does not support indexing

Fix by removing the file position logic and the associated messages
and make the file positioning the responsibility of the caller.

The port counter read function argument is changed to the per port
data structure since the counters are relative to the port and not
the device.

Reviewed-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>