libpayload: Add UNKNOWN_SPEED to usb_speed enum

xhci_rh_port_speed return -1 if the port is disabled. The usb_speed enum
is unsigned so this results in a positive value which implies success.

Adding a -1 to the enum will make it signed so the >= 0 check will work
correctly.

BUG=b:76831439
TEST=verified on grunt that -1 is returned when port is disabled.

Change-Id: I98a373717d52dfb6ca4dcc53a00dc1b4c240a919
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/27476
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
2 files changed