eal/linux: fix device monitor stop return

eal/linux: fix device monitor stop return

The ret value in rte_dev_event_monitor_stop stands for whether the
monitor has been successfully closed, and should not bind with
rte_intr_callback_unregister, so once it goes to the right exit point of
rte_dev_event_monitor, the ret value should be set to 0.

Also, the refmonitor count has been carefully evaluated, the value
change from 1 to 0, so there is no potential memory leak failure.

Fixes: 1fef6ced07f3 ("eal/linux: allow multiple starts of event monitor")
Cc: stable@dpdk.org
Signed-off-by: Wenxuan Wu <wenxuanx.wu@intel.com>