greybus: uart: fix double free of tty port

When inserting and removing a module with a UART protocol defined a
double free of the tty_port would happen and that would generate a lot
of kernel oops in different places related to memory corruption.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>