Fix CSMS not working with CSMS_16BIT_TAGS

Sending multipart SMS with the default csms_method CSMS_16BIT_TAGS will never work because of the way this comparison is set up.
`SmppClient::CSMS_PAYLOAD` is 1
`SmppClient::CSMS_16BIT_TAGS` is 0

Therefore `(!SmppClient::CSMS_16BIT_TAGS != SmppClient::CSMS_PAYLOAD)` will be false.

As far as I can see the only case where we don't need $parts later on is when `self::$csms_method == CSMS_PAYLOAD`.