serial: liteuart: clean up rx loop variables

The `status` variable will always be `1` when passed into the call
to `uart_insert_char()`, so it can be eliminated altogether.

Use `u8` as the type for `ch`, as it matches the return type of
the `litex_read8()` call which produces its value.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123130500.1030189-10-gsomlo@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>