Bluetooth: Mesh: Fix timeout handling on link opening in RPR Client

This commit fixes an issue, where the RPR Client gets stuck forever
until reboot if it doesn't receive Link Status or Link Report message
when openinig the remote provisioning link.

The RPR_CLI_LINK_OPEN flag was used to control the remote provision link
and protect against receiving an unexpected message, but it didn't cover
a case when neither of messages were received from the RPR Server.
cli->link.state only changes by Link Status or Link Report message and
is set to BT_MESH_RPR_LINK_IDLE when opening the link. Therefore, when
the RPR Client's timer timed out, the client didn't reset its internal
states.

This commit adds an additional state to cover this intermediate state
when the client initiated the link opening but didn't receive any
response.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>