Bluetooth: Controller: Use maximum Adv PDU size time reservation

Use the maximum advertising PDU size time reservation
consider the Advertising Data could be updated from zero to
maximum support size. If maximum time reservation is
disabled then time reservation corresponding to the
Advertising Data present at the time of the start/enable
of Advertising is used.

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