rust: pl011: fix warning with new clippy

Newer versions of clippy are able to see that all the variants in
the PL011 word length enum end with "Bits", and complain about it.
Allow it.

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/registers.rs

@@ -255,6 +255,7 @@ pub enum Mode {
 
 #[bitsize(2)]
 #[derive(Clone, Copy, Debug, Eq, FromBits, PartialEq)]
+#[allow(clippy::enum_variant_names)]
 /// `WLEN` Word length, field of [Line Control register](LineControl).
 ///
 /// These bits indicate the number of data bits transmitted or received in a