thunderbolt: use `tb_eeprom_get_drom_offset` to discover DROM offset

The static function `tb_eeprom_get_drom_offset` has more safety guards
for the DROM offset fetching.  Use this instead of just `tb_sw_read`

No intended functional changes.

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