staging: dgnc: cleanup properly

dgnc_cleanup_module() was called when the module unloaded to do a total
cleanup and it was also called if pci_register_driver() fails. But
dgnc_cleanup_module() will try dgnc_remove_driver_sysfiles() but the
sysfiles will be created only if pci_register_driver() succeeds.
So if pci_register_driver() fails and we try dgnc_cleanup_module() then we
were getting:

[  942.001479] BUG: unable to handle kernel NULL pointer dereference at 00000018
[  942.001482] IP: [<c122c7a8>] sysfs_remove_file_ns+0x8/0x20

with part of the call trace as:

[  942.001544] Call Trace:
[  942.001555]  [<c149acc6>] driver_remove_file+0x16/0x20
[  942.001571]  [<f864a708>] dgnc_remove_driver_sysfiles+0x18/0x40 [dgnc]
[  942.001575]  [<f8643ac7>] dgnc_cleanup_module+0x47/0x260 [dgnc]
[  942.001577]  [<f86cb000>] ? 0xf86cb000
[  942.001580]  [<f86cb1e6>] dgnc_init_module+0x1e6/0x1000 [dgnc]

Lets have a separate cleanup function which will execute
dgnc_remove_driver_sysfiles() depending on the argument passed to it.

Reported-by: Navy Cheng <navych@126.com>
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>