Bluetooth: Mesh: Fix unable sent mesh message

In PR (#58723) has introduce another bug, that,
the flag ADV_FLAG_PROXY set before actually enabled.

When ctx:: BT RX call schedule_send will atomic_test_and_clear
ADV_FLAG_PROXY, but at this time, the proxy advertising will
not at advertising state, maybe in update params or set adverting
data phase,

so that, call bt_le_ext_adv_stop will nothing, and then call
k_work_reschedule --> send_pending_adv(at this time, the proxy
advertising actually enabled, but the upper layer clear proxy flags),
cause latest advertising unable start, because unable in advertising
state to update params(-EINVAL).

Fixes: #58721

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>