staging: dgap: use remove function

the remove callback will be called in two cases, 1) if the driver is
removed and 2) if the pci device is removed. as of now all the board
cleanups were being done in the module unload section so if the module
is unloaded everything works. But if the pci device is removed then the
loaded driver will be left in an inconsistent state.
So moved the cleanups in the remove callback and since there was no
reference of dgap_driver in remove_one(), so had to define the
pci_driver in the function.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>