openvswitch: avoid warnings in vport_from_priv

This change, firstly, avoids declaring the formal parameter const,
since it is treated as non const. (to avoid -Wcast-qual)
Secondly, it cast the pointer from void* to u8*, since it is used
in arithmetic (to avoid -Wpointer-arith)

Signed-off-by: Daniele Di Proietto <daniele.di.proietto@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>