identity: Fix the ts-offset property getter

Previous code was a copy/paste from the property setter function.
22 jobs for identity-ts-offset-get in 48 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #414813
gst indent

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passed #414812
manifest

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  Build
passed #414819
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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manual #414823
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #414822
gst-macos-10.14
build cerbero macos x86_64

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passed #414814
build fedora x86_64

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passed #414815
build nodebug fedora x86_64

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passed #414816
build static fedora x86_64

00:14:37

passed #414817
build static nodebug fedora x86_64

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  Test
passed #414824
check fedora

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passed #414825
integration testsuites fedora

00:10:46

passed #414830
valgrind bad

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passed #414827
valgrind base

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passed #414826
valgrind core

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passed #414831
valgrind ges

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passed #414828
valgrind good

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passed #414829
valgrind ugly

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  Integrate
passed #414832
android universal examples

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passed #414833
documentation

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