fmcomms5: port streaming to libiio v1 multi-buffer API

[999Marius]

PR Description

Port fmcomms5 streaming to the libiio 1.0 multi-buffer API.

Changes

• Replace iio_device_create_buffer() with iio_device_get_buffer() in ad9361_fmcomms5_phase_sync.c and test/fmcomms5_sync_test.c
• Remove iio_buffer_destroy()
• Move channels mask from iio_device_create_buffer() to iio_buffer_create_stream(), for the new API design
• Fix null check on buffer: iio_err()-> !rxbuf since iio_device_get_buffer() returns NULL, not a pointer-encoded error

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 commented new code, particularly complex or unclear areas
• I have checked that I did not introduce new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[dNechita]

Looks good!