drivers/nble: Call bt_conn_security() on start pairing event

Call bt_conn_security() the same way it is done in upstream code for
NBLE. Currently use BT_SECURITY_MEDIUM as default sec level.

Makes at least following cases pass:
...
1/6   GATT   TC_GAR_CL_BI_05_C   PASS
2/6   GATT   TC_GAR_CL_BI_17_C   PASS
3/6   GATT   TC_GAR_CL_BI_22_C   PASS
4/6   GATT   TC_GAR_CL_BI_27_C   PASS
5/6   GATT   TC_GAR_CL_BI_33_C   PASS
6/6   GATT   TC_GAW_CL_BI_06_C   PASS
...

Change-Id: I2a0c177a187db931e88e2f82a5bda1c0cc04a112
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>