drivers/staging/comedi/Kconfig: Let COMEDI_II_PCI20KC depend on HAS_IOMEM

drivers/staging/comedi/Kconfig: Let COMEDI_II_PCI20KC depend on HAS_IOMEM

COMEDI_II_PCI20KC needs HAS_IOMEM, so depend on it. The related error (
with allmodconfig under um):

    CC [M]  drivers/staging/comedi/drivers/ii_pci20kc.o
  drivers/staging/comedi/drivers/ii_pci20kc.c: In function ‘ii20k_attach’:
  drivers/staging/comedi/drivers/ii_pci20kc.c:442:2: error: implicit declaration of function ‘ioremap’ [-Werror=implicit-function-declaration]
    dev->mmio = ioremap(membase, II20K_SIZE);
    ^
  drivers/staging/comedi/drivers/ii_pci20kc.c:442:12: warning: assignment makes pointer from integer without a cast [enabled by default]
    dev->mmio = ioremap(membase, II20K_SIZE);
              ^
  drivers/staging/comedi/drivers/ii_pci20kc.c: In function ‘ii20k_detach’:
  drivers/staging/comedi/drivers/ii_pci20kc.c:512:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
     iounmap(dev->mmio);
     ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>