Bluetooth: Controller: Fix ISO Tx PDU buffer counts for fragmentation

Fix ISO Tx PDU buffer count required when SDU fragmentation
is used.

Rename BT_CTLR_CONN_ISO_SDU_LEN_MAX to
BT_CTLR_ISO_TX_SDU_LEN_MAX so the value is common to both
Broadcast and Connected ISO transmissions.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>