Bluetooth: BAP: Shell: Minor refactor of LC3 encoder

This commit renames a few variables and changes the
type and adds some additional checks.

It moves stream specific data to the shell stream
struct, but still keeps the global values that were used
to initialize the LC3 encoder

The purpose of this is to better allow for a future LC3 decoder
without any clashes in names or the like.

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