staging: comedi: addi_apci_1516: update the Kconfig

This driver does not use virt_to_bus(), remove the depends on for it.

Update the Kconfig entry to list all the boards supported by this
driver. Also, expand the help text a bit.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>