Change the ADDBA buffer size attribute type to U16 from U8

Change the ADDBA buffer size attribute type to U16 from U8

The max supported ADDBA buffer size value is 256, so change
the buffer size attribute type to U16 to configure the testbed
device to use the 256 buffer size in ADDBA negotiation in 11ax
testing.

This attribute is used only to configure a testbed device and the old
definition of this attribute was not used in any deployed implementation
hence it is still justifiable to change the definition.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>