Bluetooth: Iso: Add BT_ISO_SDU_BUF_SIZE macro

Add the BT_ISO_SDU_BUF_SIZE which can be used to declare
the TX buffers for ISO, similar to the BT_L2CAP_SDU_BUF_SIZE
macro for L2CAP.

This also updates the ISO samples to use this as well
updating the SDU check to use
CONFIG_BT_ISO_TX_MTU/CONFIG_BT_ISO_RX_MTU without
subtracting the BT_ISO_CHAN_SEND_RESERVE to make the
API more clear.

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