expect: update code for Tcl channel implementation

expect: update code for Tcl channel implementation

Tcl 8.4.0 changed the channel implementation, take a patch submitted
upstream to update the code for the new interface and remove the silencing
of the incompatible assignment error which was due to this issue.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>