smscb: Fix adding of SMSCB messages when no message with lower period exists

Adding SMSCB messages to a BTS so far only worked if there were
existing messages with a lower scheduling period than the new message.

Before this patch, it fails for new messages if they are of equal or
lower scheduling period than the existing messages.

Change-Id: I69a05b22200b3a1ee406b0673553e135603d723b
1 file changed