drivers: net: ppp: asynch api: use microseconds

Updated uart_rx_enable() and uart_tx() to use timeout given
in microseconds. Previously argument was given in milliseconds.
API change was done in:
https://github.com/zephyrproject-rtos/zephyr/pull/39041



Signed-off-by: Jani Hirsimäki <jani.hirsimaki@nordicsemi.no>