Staging: comedi: fix build breakages on some platforms

Some platforms do not have virt_to_bus(), so properly depend on
CONFIG_VIRT_TO_BUS for the Comedi drivers that need this function.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>