drivers: mcux_flexcomm: fix character glitches at low power entry

Character glitches are observed when entering suspend and standby low
power modes.
To fix it, we make sure the `poll_out` API waits for the character
transfer to complete. This is aligned with the uart driver API
description.

Signed-off-by: Axel Le Bourhis <axel.lebourhis@nxp.com>