mic: vop: Fix crash on remove

The remove path contains a hack which depends on internal structures in
other source files, similar to the one which was recently removed from
the registration path.  Since commit 1ce9e605 ("virtio_ring:
introduce packed ring support"), this leads to a crash when vop devices
are removed.

The structure in question is only examined to get the virtual address of
the allocated used page.  Store that pointer locally instead to fix the
crash.

Fixes: 1ce9e605 ("virtio_ring: introduce packed ring support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>