io_edgeport: Fix various bogus returns to the tty layer

The edgeport reports negative error codes to functions that do not
expect them. This can cause ports to jam forever

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>