drivers/nble: Fix setting connection role

According to the data from nble_gap_connect_evt,
role_slave is "0 if connected as master, otherwise as slave",
so to be consistent with our API, this shall be mapped to
either BT_CONN_ROLE_MASTER or BT_CONN_ROLE_SLAVE.

Change-Id: Icf31ef02ea178e04cb28b2611e09b06c23f49704
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>