staging: dgnc: Kconfig: add dependency PCI for it

DGNC is based on PCI, so need add related dependency, or when compiled
with PCI disabled, it will be fail.

The related error (randconfig for PCI disabled under s390):

  drivers/staging/dgnc/dgnc_cls.c:1394:2: error: implicit declaration of function ‘ioremap’ [-Werror=implicit-function-declaration]
  drivers/staging/dgnc/dgnc_cls.c:1407:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]

Cc: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/Kconfig

 config DGNC
        tristate "Digi Neo and Classic PCI Products"
        default n
-       depends on TTY
+       depends on TTY && PCI
        ---help---
        Driver for the Digi International Neo and Classic PCI based product line.