zd1211rw: fix build warning

Tasklet conversion patches intoruced a build warning
in init_usb_rx(), this patch fixes the same.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 26721b02 ("zd1211rw: convert tasklets to use new tasklet_setup() API")
Signed-off-by: Allen Pais <allen.lkml@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200831043425.281538-1-allen.lkml@gmail.com