[SX1272 & SX1276 Driver] Fix Fifo Pointer in Continuous Mode

[Eric-A-Marks]

Summary of changes

In continuous mode we need to check and set the fifo pointer before every read, otherwise data may be lost.

This is bug is commonly seen when using the mbed LoRaWAN FUOTA demo as small delta updates working, but large full updates failing.

https://github.com/ARMmbed/mbed-os-example-lorawan-fuota/

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Documentation

ARMmbed/mbed-semtech-lora-rf-drivers#56

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

[ciarmcom]

@Eric-A-Marks, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

Will be merged as soon as we have 6.0.0 beta1 released