HCI_EXT_SetTxPowerCmd only works if adv sets are destroyed/not yet created

Problem Description

HCI_EXT_SetTxPowerCmd does not affect an advertisement set's tx power after
an advertisement set has been enabled. This issue does not occur before if HCI_EXT_SetTxPowerCmd is called before GapAdv_create().

The issue was seen in the simple_peripheral example after GapAdv_create()is used to enable an advertisement. Calling HCI_EXT_SetTxPowerCmd after this step does not result in a change in TxPower.

Root cause/Fix:

This issue was fixed in the link layer. HCI_EXT_SetTxPowerCmd will allow runtime changes to the TX power only if using GapAdv_create() passed in with GAP_ADV_TX_POWER_NO_PREFERENCE, which is set by default in the application.