staging: comedi: COMEDI_NI_LABPC_ISA no longer depends on VIRT_TO_BUS

After migrating ISA DMA support from the "ni_labpc" module to the new
"ni_labpc_isadma" module, the `COMEDI_NI_LABPC_ISA` configuration option
no longer depends on `VIRT_TO_BUS`, so remove the dependency.

(The new `COMEDI_NI_LABPC_ISADMA` option does depend on `VIRT_TO_BUS`
but is not configured manually and is only selected automatically if the
`VIRT_TO_BUS` and `ISA_DMA_API` options are set.)

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