fsi: Make fsi_bus_type a private variable to the core

fsi: Make fsi_bus_type a private variable to the core

There are no users of fsi_bus_type outside of fsi-core.c, so make that
variable static, don't export it and drop the declaration from the
public header file.

As there is a usage of fsi_bus_type in fsi_create_device() the
definition of that variable must happen further up in the file to not
have to add a local declaration.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Acked-by: Eddie James <eajames@linux.ibm.com>
Link: https://patch.msgid.link/bfd83034dec04d5a6b01a234988377fc6224614d.1765279318.git.u.kleine-koenig@baylibre.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>