staging: vt6656: Don't leak memory in drivers/staging/vt6656/ioctl.c::private_ioctl()

If copy_to_user() fails in the WLAN_CMD_GET_NODE_LIST case of the
switch in drivers/staging/vt6656/ioctl.c::private_ioctl() we'll leak
the memory allocated to 'pNodeList'. Fix that by kfree'ing the memory
in the failure case.
Also remove a pointless cast (to type 'PSNodeList') of a kmalloc()
return value - kmalloc() returns a void pointer that is implicitly
converted, so there is no need for an explicit cast.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>