[libata] fix smatch warning for zpodd_wake_dev

Fix a smatch warning caused by an useless pointer check.
The context parameter (aka. ata_dev) will never be NULL until we remove
the acpi notification handler, so it is pointless to check it for NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>