cw1200: make cw1200_spi_irq_unsubscribe() void

Fix the following coccicheck warning:

drivers/net/wireless/st/cw1200/cw1200_spi.c:273:5-8: Unneeded variable:
"ret". Return "0" on line 279

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/20200410090910.27132-1-yanaijie@huawei.com