netdev-linux: Cast policer rate to uint64_t to avoid overflow

tc_fill_rate() takes a 64bit int, casting kbits_rate from int
to uint64_t avoids a possible overflow when translating from
kbits to bytes.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>