staging: comedi: addi_apci_3xxx: check return value

Function pci_ioremap_bar() will return a NULL pointer if there is no
enough memory. However, in function apci3xxx_auto_attach(), the return
value of function pci_ioremap_bar() is not validated. This may result in
NULL dereference in following access to dev->mmio. This patch fixes the
bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>