dp-packet: Remove 'frame' member.

dp-packet: Remove 'frame' member.

In 'struct ofpbuf' the 'frame' pointer was used to parse different kinds of
data (Ethernet, OpenFlow, Netlink attributes).  For Ethernet packets the
'frame' pointer was supposed to have the same value as the 'data'
pointer.

Since 'struct dp_packet' is only used for Ethernet packets, there's no
need for a separate 'frame' pointer: we can use the 'data' pointer
instead.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>