of: Fix name of class member variables

Fix name of class member variables in OFPMeterFeaturesStats
to reflect OpenFlow Specification.

Accordingly, fix json pattern files for unit tests.

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