[VTA] Fix VTA function Vivado Compile Error. (#3375)

[VTA] Fix VTA function Vivado Compile Error. (#3375)

Issue:
when using vivado compile vta.cc with top function 'vta', vivado
report deadlock error like '...with default size is used in a non -dataflow
region, which may result in deadlock Please consider to resize the
stream using the directive ‘set_directive_stream’ or the ‘HL S stream’
pragma.'

Solution:
give the queue a default size as 8.