vboot_ref/flashrom_drv: Allow sentinel verbosity value

Allow for picking the default verbosity with a sentinel
value of -1.

BUG=b:207808292
BRANCH=none
TEST=none

Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Change-Id: Ia3a21e8647fe480c60ea80ecb060466e9d86110a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3461094
Tested-by: Edward O'Callaghan <quasisec@chromium.org>
Auto-Submit: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Nikolai Artemiev <nartemiev@google.com>
Commit-Queue: Nikolai Artemiev <nartemiev@google.com>