nimble/ll: Fix extended advertising report event length
nimble/ll: Fix extended advertising report event length

`struct ble_ll_ext_adv_report` contains event meta code and event
length (HCI event header) which shall not be calculated into event length.
This patch fixes it.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble