dma/idxd: fix default for workqueue options

dma/idxd: fix default for workqueue options

When no --wq-option flag is passed to dpdk_idxd_cfg.py script, the
default value used by argparse is None. This leads to later errors which
are expecting an array value:

	TypeError: 'NoneType' object is not iterable

This is fixed by specifying that the empty array should be the default
if no flags are passed.

Fixes: 94b0f04dc0e3 ("dma/idxd: add generic option for queue config")
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Sunil Pai G <sunil.pai.g@intel.com>
Acked-by: Conor Walsh <conor.walsh@intel.com>