Remove dead tuner commands (TUNERINIT/TUNERTUNE/TUNERSTDBY)

[ringof]

The SDDC_FX3 firmware no longer handles R82xx tuner commands — the
GPL-licensed tuner driver was removed in ringof/rx888-firmware#1. The
firmware now returns USB STALL for these vendor requests.

rx888_stream still references these commands:

rx888.h — enum definitions:

• TUNERINIT = 0xB4
• TUNERTUNE = 0xB5
• TUNERSTDBY = 0xB8
• R82XX_ATTENUATOR = 1, R82XX_VGA = 2, R82XX_SIDEBAND = 3, R82XX_HARMONIC = 4

rx888_stream.c:~1202 — best-effort TUNERSTDBY call during init:

/* Put tuner in standby as original did (best-effort). */
usleep(CTRL_SETTLE_US);
(void)rx888_cmd_u32(h, (uint8_t)TUNERSTDBY, 0u);

[ringof]

Proposed changes

rx888.h: Remove TUNERINIT, TUNERTUNE, TUNERSTDBY from
FX3Command enum. Remove R82XX_ATTENUATOR, R82XX_VGA,
R82XX_SIDEBAND, R82XX_HARMONIC from ArgumentList enum.

rx888_stream.c: Delete the TUNERSTDBY call (~line 1202). The
firmware STALLs it anyway; removing it saves one control transfer
and avoids a misleading libusb error at -v2.

Verification

The existing fw_test.sh tests (10-12) already confirm the firmware
STALLs these commands, so no new tests are needed. Just verify
rx888_stream builds and streams cleanly after the removal.