ssb: Fix Sparse error in main

This change fixes below sparse error:
drivers/ssb/main.c:94:16: warning: symbol 'ssb_sdio_func_to_bus'
was not declared. Should it be static?

Acked-by: Michael Buesch <m@bues.ch>
Signed-off-by: Pramod Gurav <pramod.gurav@smartplayin.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>