wlcore: spi: fix build warning caused by redundant variable

The ret variable is unused in wlcore_probe_of()
Remove it for fixing build warning.

Fixes: 01efe65a ("wlcore: spi: add wl18xx support")
Signed-off-by: Eyal Reizer <eyalr@ti.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>