wctc4xxp: Reload the firmware if a fatal alert was received.

The driver will now automatically reload the firmware when there are no open
channels if the firmware reports a fatal error. If the firmware reports an
error, but it was not fatal, it will leave things running and try to reload when
all channels are shut down. The driver will also halt channel processing and
reload the firmware if a channel ever failed to be created.

The thought is that if the DTE reports a non-fatal error, I cannot be certain
what the state is, and it should be reset when possible without impacting
otherwise functioning card.  If there are problems, presumably all users would
hang up and the driver will then reload the firmware.

If the error is fatal, then all processing is halted to encourage everyone to
hang up. The card is probably not working at this point anyway, so there is no
point in trying to communicate with it.

Also included in this change is a compile-time selectable debug sysfs attribute
that will allow forcing an alert condition for testing the recovery.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>
wctc4xxp: Constrain RTP payload to 500 bytes.

This is a limitation of the DTE firmware that normally would result in dropped
packets on the firmware. If the driver knows it is going to be dropped it should
drop it.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>
wctc4xxp: Trivial removal of unused structure members.

These were left over from recent developments and are not used by the driver.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>