bootstd: cros: store partition type in an efi_guid_t

The scan_part() function uses a struct uuid to store the little-endian
partition type GUID, but this structure should be used only to contain a
big-endian UUID. Use an efi_guid_t instead.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>