shared/usb: per-LUN response to PREVENT_ALLOW_MEDIUM_REMOVAL

Respond with "unsupported" for the SD LUN (removable media) and OK for
internal flash / SAVES LUNs (non-removable). Responding OK for a
removable LUN tells macOS the medium is always present, and macOS then
skips TEST_UNIT_READY polling. If the SD isn't ready at the single
enumeration probe, macOS never re-checks and LUN 1 fails to publish an
IOMedia node.

Hathach documented this behavior back in #6555; the SD case wasn't
differentiated at the time.

Fixes #10965.