IB/ipath: Export hardware counters more consistently

Various hardware counters are exported via the ipath file system (since
it is binary data).  The old file format was very dependent on the HW
offsets for these registers.  Newer HCA chips can have different
counters at different offsets.  This patch adds a level of indirection
to make the file format consistent across HCAs.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>