xhci: remove unnecessary check in xhci_free_stream_info()

This patch removes the unneccessary check 'if (stream_info)' because
there is already a check few lines above which ensures that stream_info
is not NULL.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>