dummyflasher.c: Replace atoi() with strtoul()

BUG=none
BRANCH=none
TEST=builds

Change-Id: Ib9e66016a2f4ce2d13b833c261f900cab80916b7
Spotted-by: Angel Pons <th3fanbus@gmail.com>
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/54854
Reviewed-by: Sam McNally <sammc@google.com>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed