This change is needed for the fs terminal to work correctly again, after
merging 44bae24c78 which shifted
responsibility of thread safety from a terminal to the DTE. (Before it
was the terminal, which removed this callback safely when processing completed,
and thus we had to check the callback return value). Not the DTE layers
are responsible for thread safety and the return value indicates
success/failure from processing.
Also uses the default clock, the same way as UART terminal.
This is for example needed when using an external crystal to select
UART_SCLK_XTAL, which fixes uart comm issue when using PM with light
sleep enabled.
