xen/xenbus: Remove the unnecessary check.

.. we check whether 'xdev' is NULL - but there is no need for
it as the 'dev' check is done before. The 'dev' is embedded in
the 'xdev' so having xdev != NULL with dev being being checked
is not going to happen.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>