pci: Address Rust 1.51.0 clippy issue (upper_case_acroynms)

warning: name `IORegion` contains a capitalized acronym
   --> pci/src/configuration.rs:320:5
    |
320 |     IORegion = 0x01,
    |     ^^^^^^^^ help: consider making the acronym lowercase, except the initial letter (notice the capitalization): `IoRegion`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#upper_case_acronyms

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

pci/src/bus.rs

@@ -122,7 +122,7 @@ impl PciBus {
     ) -> Result<()> {
         for (address, size, type_) in bars {
             match type_ {
-                PciBarRegionType::IORegion => {
+                PciBarRegionType::IoRegion => {
                     #[cfg(target_arch = "x86_64")]
                     io_bus
                         .insert(dev.clone(), address.raw_value(), size)