serial: mpc52xx: add delay after resetting transmitter to fix broken chars

This fixes receiving broken characters on the console from an MPC5125
system when systemd comes up which repeatedly opens and shuts down the
console device.

Trial and error with the needed interval showed that 500 us are good
enough most of the time when using 38400 Bd, so I think 1 ms is a good
compromise between fixing the issue and not penalize faster setups too
much.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>