Fix fan_mode + fan_mode_duration rejection in set_state

[crrapi]

This fixes a logic bug in set_state() where passing both --fan-mode and --fan-mode-duration would incorrectly raise a "only one attribute at a time" error — even though that combination is supposed to be allowed.

The issue was that fan_mode_duration was being treated as a separate top-level attribute in the mutual exclusivity check. So even when used correctly (with fan_mode), it got flagged.

What changed

• Removed fan_mode_duration from the list of mutually exclusive fields.
• The check that enforces fan_mode and fan_mode_duration must be passed together is still in place.

Why it matters

This unblocks valid usage patterns — including Home Assistant and CLI calls — that need to set both fan_mode and fan_mode_duration together.