The si32260 chip may enter a state of thermal alarm to avoid
overheating. This disables the channel.
This commit allows automatic restoring of the channel after a certain
timeout (poll_chan_linefeed, by default 30 seconds).
Adds a per-channel counter ("overheats") in
/proc/xpp/XBUS-<n>/XPD-<m>0/fxs_info to count such events.
Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>