Modify function input parameter type and event parameter type

[JLong-Song]

Hi Alexander Houben sir,

I noticed some enums was not be used, so I change some functions' input parameter type to enum,

and change some events' parameter type to enum too.

Unfortunately, I don't have any BLE device to make some unit tests for now.

Hope it's helpful to you.

changed functions:
ble_cmd_system_endpoint_tx
ble_cmd_system_whitelist_append
ble_cmd_system_whitelist_remove
ble_cmd_system_endpoint_rx
ble_cmd_system_endpoint_set_watermarks
ble_cmd_sm_set_parameters
ble_cmd_gap_set_mode
ble_cmd_gap_discover
ble_cmd_gap_set_filtering
ble_cmd_gap_set_directed_connectable_mode

changed events:
ble_msg_attributes_value_evt_t
ble_msg_attributes_status_evt_t
ble_msg_connection_status_evt_t
ble_msg_attclient_attribute_value_evt_t
ble_msg_sm_bond_status_evt_t

Best Regards,
Chieh-Lung Sung