Transceiver52M: Fix retransmissions when filler table is enabled

Commit 15d743efaf8d3ec8dacd37fbac434c1e719c0b30 "Disable filler table
retransmissions by default" made OpenBTS style filler table behavior
optional. When enabled, dummy bursts were automatically loaded into the
filler table, but the table was not updated and only filler busts were
retransmitted.

Enable the restransmit state flag when the filler table option is
specified. Only preload filler table and enable retransmissions on
channel zero.

Signed-off-by: Thomas Tsou <tom@tsou.cc>