fm10k: fix typos on fall through comments

Newer versions of GCC since version 7 now warn when a case statement may
fall through without an explicit comment. "Fallthough" does not count as
it is misspelled. Fix the typos for these comments to appease the new
warnings.

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>