Staging: fwserial: Declare fwtty_port_get as static

Declare the function fwtty_port_get as static since it is used
only in this particular file. Also remove the corresponding
declaration from header file.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>