net/dhcpv4: Move timers inside the net_if dhcpv4 context

The dhcpv4 state machine has a dedicated context within each net_if
structure.  For reasons unknown the timers used by the dhcpv4 state
machine have been placed in net_if outside of the dhcpv4 context area.
Relocate them into the dhcpv4 context.

Change-Id: I0531f493610dffda9ca9208993597a5665bde997
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>