Bluetooth: controller: llcp: update max dle times on feature exchange

On a completion of feature exchange the max DLE times needs to be
updated if Coded PHY has become supported.
For this now also keep a separate store of the default_tx_octets/time
for the sake of re-calculation of local versions of DLE values

Signed-off-by: Erik Brockhoff <erbr@oticon.com>