Bluetooth: Mesh: Release previously reserved slot before reserving again

If the previous upload was in-band and it didn't complete, the slot will
stay reserved. By design we release slot not at the end of the upload
phase, but at the start of a new upload phase.

This fixes DFU/SR/FD/BV-13-C.

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