ofctl_rest: Fix clear flow entries command

Currently, clear flow entries command (/stats/flowentry/clear/<dpid>)
in ofctl_rest deletes flow entries from only table 0.
This patch fixes the command to delete flow entries from all tables.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>