ipw2x00: make ipw_setup_deferred_work() void

This function actually needs no return value. So remove the unneeded
variable 'ret' and make it void.

This also fixes the following coccicheck warning:

drivers/net/wireless/intel/ipw2x00/ipw2200.c:10648:5-8: Unneeded
variable: "ret". Return "0" on line 10684

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200414120251.35869-1-yanaijie@huawei.com