optionrom: fix bugs in signrom.sh

signrom.sh has multiple bugs:

- the last byte is considered when calculating the existing checksum, but not
  when computing the correction
- apprently the 'expr' expression overflows and produces incorrect results with
  larger roms
- if the checksum happened to be zero, we calculated the correction byte to be
  256

Instead of rewriting this in half a line of python, this patch fixes the bugs.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
