identity: Fix the ts-offset property getter

Previous code was a copy/paste from the property setter function.
22 jobs for backport-mr-203-into-1.16-identity-ts-offset-get in 66 minutes and 6 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Preparation
passed #488901
gst indent

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passed #488900
manifest

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  Build
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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gst-ios-12.2
build cerbero ios universal

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gst-macos-10.14
build cerbero macos x86_64

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build fedora x86_64

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build nodebug fedora x86_64

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build static fedora x86_64

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build static nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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valgrind bad

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valgrind base

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valgrind core

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valgrind ges

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valgrind good

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valgrind ugly

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valgrind good

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  Integrate
passed #488920
android universal examples

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