um: chan_xterm: Fix fd leak

xterm serial channel was leaking a fd used in setting up the
port helper

This bug is prehistoric - it predates switching to git. The "fixes"
header here is really just to mark all the versions we would like this to
apply to which is "Anything from the Cretaceous period onwards".

No dinosaurs were harmed in fixing this bug.

Fixes: b40997b8 ("um: drivers/xterm.c: fix a file descriptor leak")
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>