firmware-utils: mkedimaximg: fix compiler warnings/errors

firmware-utils: mkedimaximg: fix compiler warnings/errors

mkedimaximg.c:95:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
mkedimaximg.c:116:19: error: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘off_t’ {aka ‘long int’} [-Werror=sign-compare]
mkedimaximg.c:137:19: error: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘off_t’ {aka ‘long int’} [-Werror=sign-compare]
mkedimaximg.c:186:28: error: taking address of packed member of ‘struct header’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
mkedimaximg.c:192:28: error: taking address of packed member of ‘struct header’ may result in an unaligned pointer value [-Werror=address-of-packed-member]

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