fix: disable RMT channel before deletion to prevent channel leak

[Mimsseey]

What's new (Author - fill this out)

The RMT driver requires channels to be in init (disabled) state before deletion. led_strip_rmt_del() was calling rmt_del_channel() while the channel was still enabled, causing "channel not in init state" errors and leaking the RMT channel. Also moved channel_enabled from static global to per-instance field so reinit works correctly after deinit

Changed

Removed

Verification (Author - fill this out)

• I have tested Primary device(s) (list exact HW + config):
• I have tested potentially affected devices (and/or list potential affected devices not available to you):
• I have wrapped device specific code with #ifdef CONFIG_... or similar: [ ] Yes / [ ] N/A
• I have updated Hugo Docs with any new or changed info for end users: [ ] Yes / [ ] N/A

Linked Issues

• Closes #

Checklist (Reviewer - don't fill this out)

• Code builds, flashes, and feature verified with no functional issues on listed device(s)
• Changes reviewed for unintended impact on other devices/targets

[the1anonlypr3]