Bluetooth: Audio: Fix missing path_id for unicast server

The codec->path_id supplied to the capabilities
were not being used for the endpoint codec.

Important note: This only works if there is a single
capability per codec ID per direction. This limitation
is also an existing limitation in the current
implementation of capabilities.c.

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