[build] Fix signed/unsigned division in util/zbin.c

[build] Fix signed/unsigned division in util/zbin.c

Commit b149a99 ([build] Round up SUBx deltas) introduced a
signed/unsigned issue that affects gPXE images built on 32-bit hosts.
The zbin fixup utility performed an unsigned division, which led to
.usb images with an incorrect number of sectors to load.

The issue snuck by on 64-bit hosts since uint32_t is promoted to long.
On 32-bit hosts it is promoted to unsigned long.

Modified-by: Michael Brown <mcb30@etherboot.org>
Signed-off-by: Michael Brown <mcb30@etherboot.org>