usb-misc: sisusbvga: Remove null test before calls to kfree()

This patch removes null test before calls to kfree() as kfree() can
handle null pointers safely.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>