staging: unisys: neglect to NULL rcvbuf pointer

Neglect to NULL rcvbuf pointer array could result in faults later

This problem would exhibit itself as a fault when when attempting to stop
any visornic device (i.e., in visornic_disable_with_timeout() or
visornic_serverdown_complete()) that had never been started (i.e., for
which init_rcv_bufs() had never been called).  Because the array of rcvbuf
was never cleared to NULLs, we would mistakenly attempt to call kfree_skb()
on garbage memory.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>