parport: check exclusive access before register

As of now we were starting the registration process and after the device
is registered we were checking if the device can be used by the
parport. Now lets check it first so that we do not need to go through
the registration process only to fail at the end.
The original exclusive access check at the end is still there so that we
do not get any surprises if two different process registers its device
with same parport and with exclusive access at the same time.

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