fm10k: add missing fall through comment

Newer versions of GCC starting with 7 now additionally warn when a case
statement may fall through without an explicit comment mentioning it.
Add such a comment to silence the warning, as this is expected.

Unfortunately the comment must come directly before the next case
statement, so we put it outside the #ifdef. Otherwise, the compiler
cannot properly detect it and thus the warning is displayed regardless.

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>