drivers: serial: Use microseconds to represent timeout

Updated uart_rx_enable() and uart_tx() to use timeout given
in microseconds. Previously argument was given in milliseconds.
However, there are cases when milliseconds granularity is not
enough and can significantly reduce a throughput, e.g. 1ms is
100 bytes at 1Mb.

Updated 4 drivers which implement asynchronous API. Updated
places where API was used.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>