nimble/host: Fix legacy advertising API with EXT_ADV enabled

[sjanc]

Configured advertisign instance is required in order to set advertising data when extended advertising HCI is used. Since when legacy PDU are in use we can set both AD and SCAN_RSP we can easily pre-configure selected instance just for this purpose and re-configure it before starting advertising without affecting set AD/SCAN_RSP.

[sjanc]

#AutoPTS run mynewt MESH/NODE/CFG/AKL/BV-02-C

[codecoup-tester]

Scheduled PR #https://github.com/apache/mynewt-nimble/pull/1775#issuecomment-2100341558 after 13:19:23.

[codecoup-tester]

AutoPTS Bot failed: Build and flash step failed

[codecoup-tester]

Scheduled PR #https://github.com/apache/mynewt-nimble/pull/1775#issuecomment-2100341558 after 15:54:59.

[codecoup-tester]

AutoPTS Bot results:

Failed tests

MESH MESH/NODE/CFG/AKL/BV-02-C FAIL

[sjanc]

#AutoPTS run mynewt MESH/NODE/BCN/SNB/BV-01-C

[codecoup-tester]

Scheduled PR #https://github.com/apache/mynewt-nimble/pull/1775#issuecomment-2100690719 after 16:17:45.

[codecoup-tester]

AutoPTS Bot results:

Failed tests

MESH MESH/NODE/BCN/SNB/BV-01-C FAIL