uart: uart_cpu_arm64: fix the build without FDT

[kevans]

Summary

clang 14 doesn't properly determine that we're unconditionally returning
if we have ACPI but not FDT. Workaround it for now by just returning
earlier explicitly, but drop a comment noting that it's sketchy.

[andrew]

Maybe we should create a new function that calls uart_cpu_fdt_probe and sets up di similar to uart_cpu_acpi_spcr. This will mean the code is

#if DEV_ACPI
if (uart_cpu_acpi_spcr(...) == 0)
    return (0);
#endif
#if FDT
if (uart_new_fdt_func(...) == 0)
   return (0);
#endif
return (ENXIO);

[kevans]

Switch to Andy's idea; do fdt setup in a new function entirely, just return
ENXIO if neither setup succeeds.