zd1211rw: remove needless check before usb_free_coherent()

usb_free_coherent() is safe with NULL addr and this check is
not required.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200630070404.8207-1-vulab@iscas.ac.cn