rust: pl011: always use reset() method on registers

rust: pl011: always use reset() method on registers

For CR, the ugly-ish "0.into()" idiom is already hidden within the
reset method.  Do not repeat it.

For FR, standardize on reset() being equivalent to "*self = Self::default()"
and let reset_fifo toggle only the bits that are related to FIFOs.  This
commit also reproduces C commit 02b1f7f6192 ("hw/char/pl011: Split RX/TX
path of pl011_reset_fifo()", 2024-09-13).

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