net: dhcpv6: adjust switch-case in `dhcpv6_enter_state`

For code clarity, unified switch-case usage in `dhcpv6_enter_state` to
use `break` instead of `return`.
Typically, a `break` is used in switch-case statements unless an early
return is necessary, in which case `return` is appropriate.

In this scenario, the `break` statement is the more suitable choice.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>