Bluetooth: Fix not reseting required sec level on error

If pairing failed before encryption was enabled or if enabling
encryption failed (eg due to remote device missing LTK) required
security level should be reset.

Otherwise it is not possible to re-try with setting security level.

Error reporting to application is still missing though.

Change-Id: I085e3ee116bd04304a4c4563cc40f9d40262447e
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>