tty: of_serial: unexport tegra_serial_handle_break

Tegra is only booted through device-tree now; there are no board files
left that use this function. Hence, don't export it. Move the static
inline definition into of_serial.c, so we can delete of_serial.h too.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>