Retevis RB75: Support full band TX/RX - fixes #10663

[KC9HI]

CHIRP PR Checklist

The following must be true before PRs can be merged:

• All tests must be passing.
• Commits should be squashed into logical units.
• Commits should be rebased (or simply rebase-able in the web UI) on current master. Do not put merge commits in a PR.
• Commits in a single PR should be related.
• Major new features or bug fixes should reference a CHIRP issue.
• New drivers should be accompanied by a test image in tests/images (except for thin aliases where the driver is sufficiently tested already).

Please also follow these guidelines:

• Keep cleanups in separate commits from functional changes.
• Please write a reasonable commit message, especially if making some change that isn't totally obvious (such as adding a new model, adding a feature, etc).
• Do not add new py2-compatibility code (No new uses of six, future, etc).
• All new drivers should set NEEDS_COMPAT_SERIAL=False and use MemoryMapBytes.
• New drivers and radio models will affect the Python3 test matrix. You should regenerate this file with tox -emakesupported and include it in your commit.