staging,dgap: Add dependency on HAS_IOMEM

staging,dgap: Add dependency on HAS_IOMEM

On archs like S390 or um this driver cannot build nor work.
Make it depend on HAS_IOMEM to bypass build failures.

drivers/staging/dgap/dgap_driver.c: In function ‘dgap_cleanup_board’:
drivers/staging/dgap/dgap_driver.c:457:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
drivers/staging/dgap/dgap_driver.c: In function ‘dgap_do_remap’:
drivers/staging/dgap/dgap_driver.c:694:2: error: implicit declaration of function ‘ioremap’ [-Werror=implicit-function-declaration]

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David S. Miller <davem@davemloft.net>