thunderbolt: Do not make DROM read success compulsory

The USB4 specification doesn't make any requirements that reading
a device router's DROM is needed for the operation of the device.

Other connection manager solutions don't necessarily read it or gate
the usability of the device on whether it was read.

So make failures when reading the DROM show warnings but not
fail the initialization of the router.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>