ssbi: Allow compilation as a module

The ssbi driver's read/write entry points are protected with wrappers
in the case when the driver isn't enabled.  These wrappers don't make
any sense, since a client of the SSBI bus won't work without it.  Make
these just regular functions, so that the SSBI driver can be built as
a module.

Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>