fm10k: add helper functions to set strings and data for ethtool stats

Reduce duplicate code and the amount of indentation by adding
fm10k_add_stat_strings and fm10k_add_ethtool_stats functions which help
add fm10k_stat structures to the ethtool stats callbacks. This helps
increase ease of use for future stat additions, and increases code
readability.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>