Bluetooth: ISO: Replace BT_ISO_STATE_ENCRYPT_PENDING with a flag

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
An ISO channel today has a state called BT_ISO_STATE_ENCRYPT_PENDING which isn't really a state, but rather a dependency before it can leave the BT_ISO_STATE_DISCONNECTED state.

The issue with BT_ISO_STATE_ENCRYPT_PENDING is that it blocks the CIS from connecting (and rightfully so), but that also means that an ISO channel will go

1. BT_ISO_STATE_DISCONNECTED
2. BT_ISO_STATE_ENCRYPT_PENDING
3. BT_ISO_STATE_DISCONNECTED
4. BT_ISO_STATE_CONNECTING
5. BT_ISO_STATE_CONNECTED

Instead of just

1. BT_ISO_STATE_DISCONNECTED
2. BT_ISO_STATE_CONNECTING
3. BT_ISO_STATE_CONNECTED

Describe the solution you'd like
Remove the BT_ISO_STATE_ENCRYPT_PENDING and replace it with a flag in the ISO channel instead.

Describe alternatives you've considered
The alternative would be to create yet another state, BT_ISO_STATE_ENCRYPT_OK so that the channel will go

1. BT_ISO_STATE_DISCONNECTED
2. BT_ISO_STATE_ENCRYPT_PENDING
3. BT_ISO_STATE_ENCRYPT_OK
4. BT_ISO_STATE_CONNECTING
5. BT_ISO_STATE_CONNECTED

Additional context
N/A