drivers: rf-transceiver: ad9361: Fix null pointer dereference in ad9361_rfpll_vco_init

[Xenoamor]

Pull Request Description

In FDD mode if the Tx and Rx frequencies are the same (as is the case in the example code) then a null pointer is dereferenced as the SynthLUT *tab pointer is not initialised

PR Type

• [x] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [x] I have followed the Coding style guidelines
• [x] I have complied with the Submission Checklist
• [x] I have performed a self-review of the changes
• [x] I have commented my code, at least hard-to-understand parts
• [ ] I have build all projects affected by the changes in this PR
• [ ] I have tested in hardware affected projects, at the relevant boards
• [ ] I have signed off all commits from this PR
• [ ] I have updated the documentation (wiki pages, ReadMe etc), if applies

[buha]

that makes sense, please do git commit --amend -s and add a brief description

i know in this case the title says it all but i'm more interested in the -s flag which adds the git name and email to the commit description, see 1st item in the checklist here https://analogdevicesinc.github.io/no-OS/contributing.html#submission-checklist

[buha]

/AzurePipelines run

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