thunderbolt: debugfs: Use FIELD_GET()

Use the FIELD_GET() macro instead of open coding the masks and shifts.
This makes the code more compact and improves readability as it avoids
the need to wrap excessively long lines.

Signed-off-by: Aapo Vienamo <aapo.vienamo@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>