Bluetooth: BAP: Shell: Fix assistant broadcast code

Fixes 2 issues:
1) The broadcast code would only ever get the first value, even if
the command specified up to 16 values.
2) The broadcast code is not just hex values, so hex2bin does not make
sense, and strlen + memcpy is much simpler.

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