i40e: Fix comment about locking for __i40e_read_nvm_word()

Caller needs to acquire the lock. Called functions will not.

Fixes: 09f79fd4

 ("i40e: avoid NVM acquire deadlock during NVM update")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>