Bluetooth: Controller: Add parameter to ISOALs sdu_write

For vendor datapaths that do not use a netbuffer to write SDUs
into, the callee of sdu_write currently has to keep track itself
of how much data has been written to the current SDU; This is wasteful
since ISOAL already keeps track of that. Add an sdu_written parameter
to the callback to inform the callee how much has been written to the
current SDU already so the callee can write using the correct offset

Signed-off-by: Troels Nilsson <trnn@demant.com>