i40e: avoid forward declarations in i40e_nvm.c

Move code around to get rid of forward declarations. No functional
changes.

After a plain code juggling, checkpatch reported:
total: 0 errors, 7 warnings, 12 checks, 1581 lines checked

so while at it let's address old issues as well. Should we ever address
the remaining unnecessary forward declarations within
drivers/net/ethernet/intel/, consider this change as a starting
point/reference.

As reported in [0], there would be a lot more of work to do...if we
care.

[0]: https://lore.kernel.org/intel-wired-lan/Zeh8qadiTGf413YU@boxer/T/#u
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>