ofproto-dpif: Fix GCC warning.

ofproto-dpif: Fix GCC warning.

gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-48) warned:

    ofproto/ofproto-dpif.c: In function 'bundle_send_learning_packets':
    ofproto/ofproto-dpif.c:1835: warning: dereferencing type-punned pointer
    will break strict-aliasing rules

I agree that its analysis matches what the C standard says.  This commit
fixes the problem and avoids the warning.

The assignment to 'port' isn't actually necessary.  I included it because
I like to have a variable with the correct type near the use of that type
through a "void *".  Then "grep" for that type is more effective, and the
compiler will be able to diagnose any later change to (in this case) the
type of the first parameter to send_packet().

Signed-off-by: Ben Pfaff <blp@nicira.com>