RMT API: Add missing get function: rmt_get_idle_level(). Suppress …

[phatpaul]

Contributing some improvements I required for my project.

1. Add missing get function to RMT API: rmt_get_idle_level().

2. Suppress error message from rmt_wait_tx_done() if wait_time==0 (allows for polling).

Here's my justification for the changes: #1175 (comment)

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ phatpaul
❌ Paul Abbott

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[phatpaul]

Delete this PR and create new one to change branch name and edit email address in commit to pass this CLA check. New PR is #2666