mesh: Fix key refresh procedure

Key refresh procedure was ignored on non-primary subnet.

Mesh Profile Specification v1.0.1:
3.10.4 Key Refresh procedure
"This procedure is used when the security of one or more network
keys and/or one or more of the application keys has been compromised
or could be compromised."

"It is possible to update each NetKey independently of all other
NetKeys. A Key Refresh procedure for one NetKey can be in a different
phase to another Key Refresh procedure for other NetKeys."

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>