staging: comedi: Kconfig: fix config COMEDI_ADDI_APCI_3120 dependants

A merge conflict between commits
fbfd9c8a ("staging: comedi:
addi_apci_3120: use dma_alloc_coherent()") and
aff5b1f8 ("staging: comedi: remove
comedi_fc module") left the COMEDI_ADDI_APCI_3120 config option
depending on VIRT_TO_BUS when it no longer needs to do so.  The
dependency was removed by the first commit and accidentally reinstated
by the second commit.  Remove the dependency again.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>