Bluetooth: TBS: Fix issues with lookup_inst_by_uri_scheme

The function took the uri as a `char *` but is in fact
not a null terminated string so change the type to uint8_t *.

Add a check fro uri_len == 0 before doing uri_len - 1

Changed the found check to just check the first octet rather
than the more costly strlen.

Fixed the type of the iterator

Add a break in the loop as we only need to find one ":"

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