HSI: fix ssi_waketest() declaration

The ssi_waketest() function definition causes a 'make W=1' warning
because the declaration is hidden away in ssi_protocol.c:

drivers/hsi/controllers/omap_ssi_core.c:147:6: error: no previous prototype for 'ssi_waketest'

Move it into a header file instead.

Fixes: dc7bf5d7 ("HSI: Introduce driver for SSI Protocol")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>