staging: dgnc: Remove driver-wide polling counter variable

The polling counter variable is only exposed via sysfs and has no other
purpose.

Now, since the polling shall be implemented as a board specific feature
rather than being global in the driver this counter is obsolete.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>