staging: fsl-mc: consistently use EXPORT_SYMBOL_GPL()

The bus driver is mixing EXPORT_SYMBOL()/EXPORT_SYMBOL_GPL() usage.
Change it to consistently use EXPORT_SYMBOL_GPL().

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>