libertas: make lbs_process_event() void

Fix the following coccicheck warning:

drivers/net/wireless/marvell/libertas/cmdresp.c:225:5-8: Unneeded
variable: "ret". Return "0" on line 355

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200413082022.22380-1-yanaijie@huawei.com