Bluetooth: host: adv: revert 39cb5744 to fix spurious error log

Commit 39cb5744 changed the log level from LOG_DBG to LOG_ERR in
bt_le_adv_resume(). This causes the error log
"No valid legacy adv to resume" to appear during normal connection
establishment when using bt_le_ext_adv_start(), even though the
system is functioning correctly.

Revert the change to restore the original LOG_DBG level.

Fixes: #94954

Signed-off-by: Samuel Tsai <stsai@atmosic.com>