serial: liteuart: Fix NULL pointer dereference in ->remove()

drvdata has to be set in _probe() - otherwise platform_get_drvdata()
causes null pointer dereference BUG in _remove().

Fixes: 1da81e55

 ("drivers/tty/serial: add LiteUART driver")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ilia Sergachev <silia@ethz.ch>
Link: https://lore.kernel.org/r/20211115224944.23f8c12b@dtkw
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>