Samples: Bluetooth: Fix bug in is_substring for broadcast sink

The is_substring did not work for true substrings, as it
would always compare [0] to [0], so it would return false
for the substring "BC" being in "ABC".

Removed the tolower as it is not necessary and fixes the issue.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>