MBO: Improve supported operating class generation

MBO: Improve supported operating class generation

Previously, 2.4 GHz operating class 81 was not added for US due to not
all of the channels (1-13 in this operating class) being supported.
Still, this operating class is the main operating class in the global
table for 2.4 GHz and it is the only option for indicating support for
the 2.4 GHz band channels in US.

Change the supported operating class building rules to include all
operating classes for which at least one channel is enabled. In
addition, fix the 80, 80+80, and 160 MHz channel checks (checking the
center frequency channel was failing since it is not a valid 20 MHz
channel).

Signed-off-by: Jouni Malinen <j@w1.fi>