ftmod_misdn: Fix brown paperbag logic bug in handle_b_channel_event()

Yeah, right, discard the incoming audio data when the b-channel is active...

Reverse the check, so ftdm_read() on an active mISDN b-channel will
be able to read audio data from the RX socketpair and not kill
the call from repeatedly timing out in poll().

Fixes:
    [WARNING] mod_freetdm.c:775 Too many timeouts while waiting I/O in channel FreeTDM/1:1/XX device 1:1!
    [ERR] mod_freetdm.c:820 clearing IO in channel FreeTDM/1:1/XX device 1:1!

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>