staging: unisys: visorbus: remove EXPORT_SYMBOL_GPL for visor_check_channel

Removing EXPORT_SYMBOL_GPU from visor_check_channel since it is
used only in visorbus.

Signed-off-by: Sameer Wadgaonkar <sameer.wadgaonkar@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>