firmware-utils: xorimage: fix compiler warnings/errors

firmware-utils: xorimage: fix compiler warnings/errors

xorimage.c:113:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
xorimage.c:142:21: error: pointer targets in passing argument 1 of ‘xor_data’ differ in signedness [-Werror=pointer-sign]
xorimage.c:142:29: error: pointer targets in passing argument 3 of ‘xor_data’ differ in signedness [-Werror=pointer-sign]
xorimage.c:145:21: error: pointer targets in passing argument 1 of ‘xor_data’ differ in signedness [-Werror=pointer-sign]
xorimage.c:145:29: error: pointer targets in passing argument 3 of ‘xor_data’ differ in signedness [-Werror=pointer-sign]
xorimage.c:64:14: error: unused variable ‘v2’ [-Werror=unused-variable]
xorimage.c:64:10: error: unused variable ‘v1’ [-Werror=unused-variable]
xorimage.c:64:6: error: unused variable ‘v0’ [-Werror=unused-variable]

Signed-off-by: Petr Štetiar <ynezz@true.cz>