i40e: fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/net/ethernet/intel/i40e/i40e_nvm.c:254:13: warning:
 symbol 'i40e_write_nvm_aq' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Tested-By: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>