rust: pl011: Rename RX FIFO methods

rust: pl011: Rename RX FIFO methods

In preparation of having a TX FIFO, rename the RX FIFO methods.
This is the Rust version of commit 40871ca758cf ("hw/char/pl011:
Rename RX FIFO methods").

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>