thunderbolt: debugfs: Fix uninitialized return in counters_write()

If the first line is in an invalid format then the "ret" value is
uninitialized.  We should return -EINVAL instead.

Fixes: 54e41810 ("thunderbolt: Add debugfs interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>