wlcore: spi: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/ti/wlcore/spi.c:87:34: warning:
 symbol 'wilink_data' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>