serial: tegra: clean up tty-flag assignments

The tty break and error flags are not bit masks so do not to use bitwise
OR when assigning them.

Note that there is no functional change due to the if-else construct and
flag having been initialised to zero (TTY_NORMAL).

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>