ena: remove surplus NULL checks when freeing ENA resources

Calling free on a NULL pointer is valid, as appropriate check is already
done internally:

/* free(NULL, ...) does nothing */
if (addr == NULL)

return;

Submitted by: Artur Rojek <ar@semihalf.com>
Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.