thunderbolt: Fix DROM handling for USB4 DROM

DROM for USB4 host/device has a shorter header than Thunderbolt DROM
header. This patch addresses host/device with USB4 DROM (According to spec:
Universal Serial Bus 4 (USB4) Device ROM Specification, Rev 1.0, Feb-2021).

While there correct the data_len field to be 12 bits and rename
__unknown1 to reserved following the spec.

Signed-off-by: Gil Fine <gil.fine@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>