rtlwifi: fix build warning

Tasklet conversion patch introduced a build
warning in _rtl_usb_init_rx, this patch fixes the warning.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: d3ccc14d ("rtlwifi/rtw88: 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/20200831043659.281712-1-allen.lkml@gmail.com