drivers: usb-misc: sisusbvga: remove dead code

The condition modex % 16 cannot be true when modex value is equal to 640
The condition du & 0xff cannot be true when du value is equal to 0x1400

Addresses-Coverity-Id: 101163
Addresses-Coverity-Id: 744373
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>