Single `ret` on qemu-virt — qemu's NS16550A model self-initialises.
Body is target-conditional via .ifdef; the C6 arm has a .error so a
build for that target fails loudly until the real register sequence
(peripheral clock, divisor, 8N1, RX IRQ enable) is written.
Static checks: function exists, body is exactly one ret, no
callee-saved writes.
