The priority flow control statistics are laid out in the stats structure
using arrays. This made it unwieldy to use as part of an i40e_stats
array.

Add a new structure type, i40e_pfc_stats, and a helper function
i40e_get_pfc_stats which can return the stats for a given priority
value as an i40e_pfc_stats structure.

Use this to create an i40e_stats array, which we'll use to format and
copy the strings and stats into the supplied buffers.

This reduces even more boiler plate code in i40e_get_ethtool_stats and
i40e_get_stat_strings.

An alternative would be to modify the structure definition for the pfc
stats, but this is more invasive to the rest of the code base.

Note that a macro was used to setup the copy of stats from the
pf->stats, as this reduces the chance of typos in the code names. It
will produce a checkpatch.pl warning due to re-use of a macro argument.
In this case, it should be safe, as the macro will fail to compile in
cases where the argument is not a simple structure member name, and thus
arguments with side effects should not be an issue.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>