Reinterpret base for meter band types bitmap.

OpenFlow 1.3 says that the band_types member of struct ofp_meter_features
is a bitmap of OFPMBT_* values.  The OFPMBT_* values are 1-based, so
until now, to avoid wasting bit 0, OVS mapped an OFPMBT_* with value 1 to
bit 0, value 2 to bit 1, and so on.  However, according to
http://openvswitch.org/pipermail/dev/2013-July/029666.html,
other OpenFlow implementations directly translate value 1 to bit 1,
value 2 to bit 2, and so on.  This commit changes Open vSwitch to use this
more common interpretation.

A request for clarification of this issue in the OpenFlow standard has been
filed with the ONF Extensibility Working Group as issue EXT-345.

Reported-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>