iio: adc: adrv9002: fix frequency hopping table configuration

[nunojsa]

PR Description

Fix multiple issues in the frequency hopping bin table write function:

• Correct typo "bellow" to "below" in comment
• Fix off-by-one error in entry bounds check (should be >= not >)
• Use correct rx20_if variable for rx2OffsetFrequencyHz instead of rx10_if
• Use correct rx2_gain variable for rx2GainIndex instead of rx1_gain

These fixes ensure proper configuration of RX2 channel parameters and prevent potential buffer overflow when the entry index equals max_sz.

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have tested the changes on the relevant hardware
• I have updated the documentation outside this repo accordingly (if there is the case)