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>

drivers/net/ethernet/intel/i40e/i40e_nvm.c

@@ -251,7 +251,7 @@ i40e_status i40e_read_nvm_buffer(struct i40e_hw *hw, u16 offset,
  *
  * Writes a 16 bit words buffer to the Shadow RAM using the admin command.
  **/
-i40e_status i40e_write_nvm_aq(struct i40e_hw *hw, u8 module_pointer,
+static i40e_status i40e_write_nvm_aq(struct i40e_hw *hw, u8 module_pointer,
 				     u32 offset, u16 words, void *data,
 				     bool last_command)
 {