HSI: fix omap ssi driver dependency

The SSI protocol implementation has an incorrect dependency on the OMAP_SSI
driver, which allows SSI to be built-in while the underlying OMAP_SSI
implementation is a loadable module, causing a link error.

This changes the dependency to the simpler 'depends on OMAP_SSI' that also
ensures that SSI-protocol can only be a module if OMAP_SSI is not built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>