batman-adv: Avoid sizeof on flexible structure

The batadv_dhcp_packet is used to read in parts of the DHCP packet and
extract relevant information for the distributed arp table. But the
structure contained the flexible member "options" which is no where used in
the code.

A sizeof on this kind of type would return the size of everything except
the flexible member. But sparse will detect this kind of sizeof and warn
with

  warning: using sizeof on a flexible structure

This can be avoided by dropping the unused flexible member.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>