net: l2: wifi: Fix roaming fail issue

The NET_EVENT_WIFI_SIGNAL_CHANGE was not added to net mgmt event queue
so that no scan was triggered, which caused roaming fail. The event
NET_EVENT_WIFI_SIGNAL_CHANGE was dropped because it was not enabled in
WIFI_SHELL_MGMT_EVENTS. After adding NET_EVENT_WIFI_SIGNAL_CHANGE, the
roaming works as expected.
Same issue found on event NET_EVENT_WIFI_NEIGHBOR_REP_COMP for 11k
roaming. Add this event to WIFI_SHELL_MGMT_EVENTS, too.

Signed-off-by: Hui Bai <hui.bai@nxp.com>