Bluetooth: SMP: Fix assumption of Central always being Initiator

SMP initiator role shall be considered the one that initiates the
pairing procedure with SMP_CMD_PAIRING_REQ:

BLUETOOTH CORE SPECIFICATION Version 5.3 | Vol 3, Part H
page 1557:

Figure 2.1: LE pairing phases

Note that by sending SMP_CMD_SECURITY_REQ it doesn't change the role to
be Initiator.

Link: bluez/bluez#567
Fixes: b28b494 ("Bluetooth: Add strict checks for allowed SMP PDUs")
Signed-off-by: Luiz Augusto von Dentz <[email protected]>