cdc_host: fix undeclared 'idx' when only LINE_CONTROL_ON_ENUM is defined

[andrewleech]

A small compile fix in the CDC host on-enum line-state handling.

If you build the CDC host with CFG_TUH_CDC_LINE_CONTROL_ON_ENUM set but CFG_TUH_CDC_LINE_CODING_ON_ENUM unset, it doesn't compile. set_line_state_on_enum() declares idx only under CFG_TUH_CDC_LINE_CODING_ON_ENUM, but it's also used in the ENUM_SET_LINE_CONTROL case (guarded by CFG_TUH_CDC_LINE_CONTROL_ON_ENUM), so idx is left undeclared in that branch.

I ran into it while looking at turning on CFG_TUH_CDC_LINE_CONTROL_ON_ENUM for a host to assert DTR/RTS on enumerate without forcing a fixed line coding, which seems like a pretty reasonable combination but currently won't build.

Fix just guards the declaration under either macro:

#if defined(CFG_TUH_CDC_LINE_CODING_ON_ENUM) || defined(CFG_TUH_CDC_LINE_CONTROL_ON_ENUM)
  const uint8_t idx = get_idx_by_ptr(p_cdc);
#endif

Testing: compile only so far. Built the CDC host class with all four combinations of the two macros (neither, coding only, control only, both); control-only failed before this change and builds after. Haven't runtime-tested the control-only enum path.

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[Copilot]

Pull request overview

Fixes a compile error in the CDC host enumeration path when CFG_TUH_CDC_LINE_CONTROL_ON_ENUM is enabled without also enabling CFG_TUH_CDC_LINE_CODING_ON_ENUM, by ensuring idx is declared whenever either enum-time line-state feature is compiled in.

Changes:

• Broaden idx declaration guard to cover both CFG_TUH_CDC_LINE_CODING_ON_ENUM and CFG_TUH_CDC_LINE_CONTROL_ON_ENUM.

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