[media] tuner-core: handle errors when getting signal strength/afc
[media] tuner-core: handle errors when getting signal strength/afc

If those callbacks fail, it should return zero, and not a random
value. The previous code assumed that all drivers would only change
signal strength if it succeeds, but this may not be true.

Signed-off-by: Mauro Carvalho Chehab <ad86ba2154032c9f55743a190faa2459a9d61d42@redhat.com>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas