samples: mesh: nrf52: minor improvement in MOVE msg handlers

Changes in MOVE message handler are  as per Mesh Model
Specification which says:

"Upon receiving a Generic Move Set message, the Generic Level
Server shall respond with a Generic Level Status message.
The target Generic Level state is the upper limit of the
Generic Level state when the transition speed is positive,
or the lower limit of the Generic Level state
when the transition speed is negative."

Signed-off-by: Vikrant More <vikrant8051@gmail.com>