Fix a bug when using zaptel timing for playing back files that have a sample rate

Fix a bug when using zaptel timing for playing back files that have a sample rate
other than 8 kHz.  The issue here is that format modules give a "whennext" sample
value, which is used to calculate when to set a timer for to retrieve the next
frame.  However, the zaptel timer operates on 8 kHz samples, so this must be taken
into account.

(another part of issue #12164, reported by milazzo and jsmith, patch by me)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@110303 65c4cc65-6c06-0410-ace0-fbb531ad65f3